Error Codes: List of most important SMTP error codes

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Error Codes: List of most important SMTP error codes

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SMTP is responsible for sending out your messages. So if you get SMTP error message, it means your e-mails were not send. It is very important you to understand why this has happened so that you can fix the problem. All SMTP codes consist of three digits, for example, 421 450, 550, etc. Not all of them mean some type of error. In order to understand how these codes work, you have to know that each digit (the first, the second and the third) have there own meaning.

The first digit tells you if your command was accepted and processed. There are five different values for that:


Mail server has accepted the command, but does not yet take any action. A confirmation message is required.


Mail server has completed the task successfully without errors.


Mail server has understood the request, but requires further information to complete it.


Mail server has encountered a temporary failure. If the command is repeated without any change, it might be completed. Try again, it may help!


Mail server has encountered a fatal error. Your request can't be processed.

As you can see, the codes that start with 4 and 5 are the ones that tell you that your message won't be sent until you find and fix the problem.

The second digit tells you more:


Syntax error


Information reply (for example to HELP request)


This digit refers to the status of connection


This digit refers to the status of the mail server

Digits 3 and 4 are not used.

The third (last) digit of the code tells you the details of mail transferring status.

Here is the list of most important SMTP error codes:


SMTP Status Codes

What the message may mean


101 or  1.0.1 – Cannot open Connection


SMTP Error 101 : Typically your SMTP server or email

program is unable to even start an SMTP session. Typical

replies will be “SMTP Error 101, Error opening connection” or “SMTP Error 101, cannot open SMTP stream”.


All SMTP Error 101 errors usually point to a configuration

problem, such as an incorrectly spelt SMTP server, or an

IP address that does not exist, or an SMTP port that does

not exist or which the recipient will not accept SMTP

connections on, or some other process is already using the default SMTP port, port 25.


111 or 1.0.1 Connection refused


SMTP Error 111 : Typically from Linux based email

systems such as SquirrelMail and Mailman. The message

will usually go like this : “Connection refused, 111 Can’t

open SMTP stream”.

All SMTP Error 111 errors usually point to an inability of

your server to communicate with the remote SMTP server

(either the recipient’s SMTP server or your ISP’s SMTP

server) or to a Linux/SMTP software configuration problem, typically /etc/hosts not being world readable, or a newly installed or reconfigured firewall preventing

connection to the remote SMTP server, or incorrect

hostnames and/or domains (e.g. does your sending

hostname match your IP address in a reverse lookup?), or

exim not running.


211 or 2.1.1 – System Status message or System

Help Reply


SMTP Error 211 : SMTP status 211 prefaces a message

about the Mail Server status or a System Help reply to the

user requesting help information. You might for example

issue a command to the mail server to display a list of

commands you can use and the server replies with an

SMTP Reply 211 followed by the list you requested.


214 or 2.1.4  – Help Reply message


SMTP Error 214 : SMTP status 214 is usually in reply to

the “HELP” command. It displays information about the

server, usually a URL to the FAQ page of the SMTP

software running on the server. As a result this “error” is

normally called a reply, as in SMTP Reply 214.


220 or 2.2.0 – <Server Name> service is running


SMTP Status 220 : This is normally the first message you

will get back from the server. It means the mail service is

running (ie. your mail server is running). It will normally

contain a welcome message and/or the title of the SMTP

software and, sometimes, the version number of the mail

server software. SMTP Reply 220 is effectively a “Hi

There, I have just this second finished starting up – I

am ready to go and at your command” informational



221 or 2.2.1 – The domain service is

closing the transmission channel


SMTP Error 221 : The server is ending the mail session –

it is closing the conversation with the ISP as it has no more mail to send in this sending session. SMTP Status 221 is often misconstrued as an error condition, when it is in fact nothing of the sort. The mail server is simply telling you that it has processed everything it was given in this particular session, and it is now going back into waiting mode. Because SMTP status 221 is often misinterpreted, with some mail servers the Network Administrators have

changed the default text of SMTP Reply 221 to something

more meaningful and less alarming. For example, a typical SMTP reply 221 might say “221 Goodbye” or

“221 Closing connection”, or the most irritating one we’ve

seen “221 Bye”, Arrrgghh – can you blame anyone for

thinking there might be a problem ? Of course not! So

some Network Administrators are these days being quite

imaginative by changing the default text of SMTP reply 221 to more user friendly messages like : “221 Thank you for your business” (I love that one!), or “221 All messages

processed successfully in this session, SMTP connection

is closing”.


250 or 2.5.0 – Requested mail action OK completed


SMTP Status 250 : The mail server has successfully

delivered the message! This is the best SMTP reply (250)

to receive - your message has been accepted and

transmitted OK !


250 is effectively a status code rather than an error code –

there is no such thing as an SMTP error 250.


251 or 2.5.1 – User not local will forward


SMTP Status 251 : The email account is not local to the

ISP server but the ISP server will accept the email and will

forward it (the server will RELAY your message, this is the most common action for ISP Mail servers – the recipient will see your ISP in the mail header as one of the first hops on the way to the recipient’s email system).

SMTP Error 251 is therefore more of an informational

message for technicians tracking how a message reached

its destination.


252 or 2.5.2  – Cannot VRFY (verify) the user – the server will accept the message and attempt

to deliver it


SMTP Status 252 : The user account appears to be valid

but could not be verified, however the server will try do

deliver the message.


There are sometimes circumstances where an email

address appears to be valid but cannot be verified as

definitely valid during the SMTP session between the

sending server (your server) and the next server to accept

your message. This can happen for example in very large

corporation where the first email receiving server might

only be an email exchanger server, a gateway server to

the eventual server which holds the user mailboxes and

which can verify if the intended recipient exists in that

organization. When this happens the gateway server will

reply with an SMTP Error 252 telling your sending server

that it cannot verify the user part of the email address, that

the domain part is OK, and that it will forward your email to a server which can do the checking and eventually deliver to the user mailbox if it exists.


354 or 3.5.4 – Start mail input end with <CRLF>.<CRLF>, or, as a less cryptic

description – “FROM and TO information

received, now please provide message

body and mark its end with <CRLF>.<CRLF>”


SMTP Error 354 : This is normally in response to the

DATA command. The server has received the From and

To information and is now asking for the “Message

Body”, the main part of the message which should be

ended by two blank lines separated by a dot (period).

Therefore, on receiving an SMTP Reply 354 the sending

server should send the body of the message to the

receiving server and indicate the end of the message body with <CRLF>.<CRLF> (note the full stop between the two Carriage_Return-Line_Feed’s).


420 or 4.2.0 – Timeout communication

problem encountered during transmission.

This is a Novell GroupWise SMTP



SMTP Error 420 : In our experience only Novell

GroupWise servers use this error (we use GroupWise!).

You will get a GroupWise GWIA (GroupWise Internet

Agent) 420 TCP Write Error or 420 TCP Read Error if

there are communication problems during transmission of

the actual message after the sending and receiving servers have actually connected. A small number of 420 SMTP errors is normal as occasional peaks of Internet usage may delay the transmission of an email with attachment so much that a timeout occurs. When a timeout occurs on a GWIA send, the message is queued up in the <Domain>\WPGATE\DEFER directory for processing at a later time (as defined in ConsoleOne or GWIA.CFG).


If you experience 420 errors only with specific recipient then it is quite likely that the recipient’s antispam firewall does not like your server, your server’s external IP address, or that your server’s HELO command uses an outbound identification that does not match your server’s external IP address (check that your sending domain’s DNS is set up correctly). In an ideal world a well behaved recipient server should really be issuing your GroupWise server with a 554 error rather than timing out and causing the GroupWise GWIA to fault with a 420 error.


If you experience too many 420 errors with all email

communications, then you have a physical

communication problem somewhere. This could be

your server’s network card, the network point that your

server is plugged into, your switch(es), your router(s),

your firewall, or your Internet line – problems caused by

routers with different MTU sizes is a classic issue. Unless

the logs of all those various problem points can give you

an instant answer, the only way you will get to the bottom

of the problem is to use a packet tracing and inspection

program like Ethereal or Wireshark, its successor, if

you’re running GroupWise on a dialog boxs or Linux server;

on NetWare your only choice is PacketScan which you

can get here

In the final analysis, if the tracing of packets, and the

changing of hardware does not help then do not discount a slightly faulty hard disk being the cause of all your

problems (even if your RAID controller or your hard disk

testing software does not detect any problem!).


421 or 4.2.1–

<Server name> Service not available – the sending email program should try again later


SMTP Error 421 : The Mail transfer service is unavailable

because of a transient event. SMTP reply 451 can be

caused by many things but generally indicates that the mail server which returns this status code is currently

unavailable but may be available later. For example, the server administrator may have stopped the mail service to troubleshoot a problem, or the mail server is right in the middle of rebooting, or the mail server is currently processing too many incoming messages or incoming requests, etc.... Note : “Mail Server” in this case can be any of the mail servers on the message’s route – the sending server (your server), the ISP SMTP server, or the recipient’s mail server. Clearly, if you repeatedly receive an SMTP status 421 then the problem is no longer of a transient nature and you need to investigate or inform the relevant network administrator, ISP tech support, or the recipient.


422 or 4.2.2 – The recipient’s

mailbox is over its

storage limit


SMTP Error 422 : Either the recipient’s mailbox is over its

storage limit or the message delivery directory (folder) on

the recipient’s mail server is currently over a size limit

imposed by the Network Administrator (e.g. possibly as a

result of the mail server having been down for some time,

having been repaired, and currently in the process of

collecting thousands of queued up messages).


431 or 4.3.1 – The recipient’s mail server is experiencing a Disk Full condition


SMTP Error 431 : The recipient’s mail server is

experiencing a Disk Full error condition, or an Out of

Memory (too many file handles) error condition (Microsoft



432 or 4.3.2 – The recipient’s Exchange Server

incoming mail queue has been stopped


SMTP Error 432 : This is an SMTP status response

specific to Microsoft Exchange Server. It indicates that the

recipient’s mail queue on their Exchange Server has been

stopped (frozen), probably while the Network Administrator troubleshoots some problem.


441 or 4.4.1 – The recipient’s server

is not responding


SMTP Error 441 : This is an error emanating from your

server indicating that the recipient’s server is not

responding. Your server will automatically try again a

number of times – how many depends on how your server

has been configured.


442 or 4.4.2 – The connection was dropped during



SMTP Error 442 : Your server started delivering the

message but the connection was broken during

transmission. This may be an unusual transient error –

however, if it keeps happening you should investigate

possible problems with your server’s network card, your

Internet routers, processes hogging the resources of your

server, and anything else which could result in a network

connection being broken.


446 or 4.4.6 – The maximum hop count was exceeded

for the message


SMTP Error 446 : The maximum hop count was

exceeded for your message. The most likely cause of this

error status code is that your message is looping internally

on your server, internally between two of your

organisation’s servers, or, sometimes, looping between

your server and the recipient’s server.


447 or 4.4.7 – Your outgoing

message timed out.


SMTP Error 447 : Your outgoing message timed out

because of problems with the receiving server who

objected to your message. Typically there is a problem

with the message header (such as too many recipients, in

most cases, or a protocol timeout between the two



449 or 4.4.9 – Routing error


SMTP Error 449 : This is a Microsoft Exchange Server

specific error code. As per Microsoft’s documentation this

error code is returned when either of the following

conditions occurs : an SMTP connector is configured to

use DNS without a smart host and also uses a non-SMTP

address space (e.g. X.400), or A message was sent to a

recipient who was identified as a member of a routing

group that was deleted.


Microsoft recommends using the WinRoute tool to

troubleshoot this error (Microsoft Knowledgebase article



450 or 4.5.0 – Requested action not

taken – The mailbox

was unavailable at the remote end. A secondary SMTP

error code may follow

“450” to refine the reason for the failure

to transmit the

message, e.g.

“SMTP Error 450


SMTP Error 450 : The server could not access the

mailbox to deliver the message. This could be caused by

a process on the remote server tidying up the mailbox, or

the remote mailbox could be corrupt, or the remote mailbox may be stored on another server which is currently offline, or the network connection went down while sending, or the remote mail server does not want to accept mail from your server for some reason (IP address, blacklisting, etc..).


In general SMTP Error 450 is a transient error at the

remote end (the destination), or at one of the routers

or servers en route to the remote end, and should

induce your mail server to retry after it’s preset retry

interval. Example of an SMTP Error 450 reply message :

“450 Please try again later”, or a classic Novell GroupWise 450 status message : “The message that you sent has been delayed. The reason given for the delay: 450 Host down (”.


SMTP Error 450 is often followed by a second SMTP error code to refine the reason for the email not reaching its destination. For example : “SMTP Error 450 5.2.3 Msg

Size greater than allowed by Remote Host”. When that is

the case and If the error message is not as clearly worded

as in this example, then simply search this document for

the secondary error code. In this case searching this

document for SMTP Error 523 or SMTP Error 5.2.3 would

yield an explanation identical to the wording above.


451or 4.5.1 – Requested action

aborted – Local error

in processing.


SMTP Error 451 : The action has been aborted by the

ISP’s server. “Local” refers to the ISP’s server. This

error is usually due to overloading at the ISP from too

many messages or transient failures. Typically some

[hopefully] temporary event prevents the successful

sending of the message. The next attempt to send by your

server may prove successful.


If this error keeps occurring to the point that it has

effectively lost its transient nature and has become

..... frequent (!!), then the problem is at your end and

you should check your own mail server (if you email out of

a corporate network), communications on your side (router, server network card), or inform your ISP if your mail server relays through your ISP or if you are a home user emailing out through Outlook, Outlook Express, dialog boxs Mail, or similar email program. Example of typical SMTP Error 451 return messages :

“SMTP error 451 Unable to complete command, DNS not

available or timed out” or “451 Domain of sender address

does not resolve” or “451 Error getting LDAP results in

map”, or “451 4.7.1 Greylisting in action, please come

back in 00:02:00 [minutes]”.


452 or 4.5.2 –   Requested action not

taken – Insufficient



SMTP Error 452 : The ISP server’s disk system has run

out of storage space, so the action had to be cancelled.

Unless you are with an ISP which is so slack that they

have not implemented Disk Full Alerts, this error usually

indicates that your ISP’s mail server is overloaded from too many messages. This can happen even to the best ISPs when, for example, there have been problems and none of the ISP’s customers could send mail; as soon as the problems are fixed there is almost always a situation where thousands of users and organizations are trying to send mail all at the same time, and those numbers can

occasionally result in the ISP’s mail servers’ hard disks

temporarily filling up, with SMTP Error 452 being the result. The next attempt to send by your server may prove



SMTP Error 452 : Most ISPs mail servers impose a

maximum number of concurrent connections that client’s

mail servers can attempt to make, and they usually also

have a limit on the number of messages that are sent per

connection. With business customers these maximums

are rarely reached, if ever. Nevertheless, If you have a lot

of messages queued up, for example as a result of the

connection to your ISP going down for a significant amount of time (and you have hundreds of users in your

organization, or it happened just as you were about to

send that large mailshot!), there could be a situation where the output of messages from your server goes over the maximum number of messages per connection allowed by your ISP. This is another case where the ISP’s server may issue a 452 SMTP error. As above, the next attempt to send by your server may prove successful.

SMTP Error 452 : This error can also be indicative of a

problem on your own mail server. Here is an example of

an SMTP 452 error : “”452 Out of memory”


465 or 4.6.5 – Code Page

unavailable on the recipient server


SMTP Error 465 : This is an Exchange Server-specific

error code. This error is returned by the recipient’s server

if the incoming email specifies a Code Page that is not

installed on the recipient’s server, normally because not all language files were installed on the server during either the installation of dialog boxs or of Exchange Server.


471 or 4.7.1 – This is a local error with the sending server and is often

followed with “Please try again later”


SMTP Error 471 : This is always a local error with your

own mail server. SMTP Error 471 (or 4.7.1) is usually

tagged onto a primary SMTP error code, for example

“SMTP Error 450 4.7.1”, or “SMTP Error 451 4.7.1”, or

“SMTP Error 550 4.7.1”; example : “451 4.7.1 Greylisting

in action, please come back in 00:02:00 [minutes]”. In all

the cases that we have seen SMTP Error 471 is usually

caused by anti-spam or virus scanning software on your

server (the sending server) getting into problems through a bug in the software, or because of a bad automatic update from the antivirus/anti-spam manufacturer, because of lack of memory on your server, or because of hard disk problems.


500 or 5.0.0 – Syntax error

command not



SMTP Error 500 : The last command sent by your server

was not recognized as a valid SMTP or ESMTP command, or is not formatted in the way the server expected. This includes situations where the command is too long.


Note that commands that are recognized, but not

implemented, are handled by different status messages

(see 502 and 504).


501 or 5.0.1 – Syntax error in

parameters or

arguments (e.g.

invalid email address) Can sometimes also be indicative of




SMTP Error 501 : The command was correct and

recognized, but the parameters (the arguments, e.g. email

address) were not valid. For example, the following email address will definitely give an SMTP Error 501 with most mail servers, happy\, as “\” is not allowed in email addresses, which makes this email address invalid.


In the vast majority of cases SMTP Error 501 is caused by invalid email addresses. For example, a typical return

error message might be : “<remote-server-ip-address>

does not like recipient. Remote host said: 501 Invalid



In cases where the error is not caused by an invalid email

address, an SMTP Error 501, particularly if repeated, can

be indicative of communications problems, such as a noisy line, intermittent drops in network connections, etc...


502 or 5.0.2 – Command not



SMTP Error 502 : The command or function issued by

your mail server is valid but has not been activated

(typically, it is not supported on this particular server).


503 or 5.0.3  – Bad sequence of



This mail server




SMTP Error 503 : In the original standards SMTP Status

503 indicates that the commands have been sent in the

wrong order, for example your mail server has sent the

“Hello” command before sending the “Mail” command.

This can often be caused by a drop in network connection

just as your server was sending a command, resulting in

the ISP’s server not receiving it and consequently not

understanding the command that followed it.

Note : this error, particularly if repeated, can be indicative

of communications problems, such as a noisy line,

intermittent drops in network connections, etc...


SMTP Reply Code 503 is nowadays more often an

indicator that the SMTP server you are trying to use

requires authentication and you tried to send a message

without authentication (username + password). This

SMTP Error 503 is permanent in that the SMTP server will

not log any errors in its log and it will not retry – you will

have to resend the email using authentication. Example of

such an error : “SMTP Error (state 13): 503 This mail

server requires authentication when attempting to send to

a non-local e-mail address. Please check your mail client

settings or contact your administrator to verify that the

domain or address is defined for this server.”.


504 or 5.0.4 – Command parameter

not implemented.


SMTP Error 504 : The command and parameter are both

valid, but the parameter is not implemented on the ISP

server, or an additional parameter or action is missing.

For example, an often encountered SMTP Error 504 is :

“504 Need to authenticate first”.

If you are receiving this error in a Microsoft

Exchange Server environment where the error shows up

in the Application Log as Event ID 7004 or 7010, then

read this Microsoft Knowledgebase article


510 or 5.1.0 – Bad Email Address


SMTP Error 510 : Bad email address. This status code

is generated by the sender’s local mail server.

If the email was addressed internally, then it means that

the addressee, as written in the email’s TO, CC, or BCC

fields, does not exist in your organization’s email system.

If the email was addressed externally, then the recipient’s

email address was misspelt.


511 or 5.1.1 – Bad Email Address


SMTP Error 511 : Bad email address. This error is

similar to error 510 and as with error 510, this status code

is generated by the sender’s local mail server.

If the email was addressed internally, then it means that

the addressee, as written in the email’s TO, CC, or BCC

fields, does not exist in your organization’s email system.

If the email was addressed externally, then the recipient’s

email address was misspelt.


512 or 5.1.2 – The host server for the recipient’s

domain name cannot be found (DNS error)


SMTP Error 512 : This SMTP reply code is received

when one of the servers on the way to the destination is

unable to resolve the domain name of a recipient email

address. Said differently : one of the servers on the way

to the destination, including your server or your ISP, has a

DNS problem or, possibly correctly, does not like one of

the email addresses in the message’s TO, CC, and BCC



The first check you should perform to resolve a 5.1.2 reply

code is to check all the recipient email addresses for

incorrect domain names (misspelt domain names, or,

maybe, totally non-existent domain names) – remember,

error code 512 is very specifically an error with the

domain name of one of the recipient email addresses.

You can call the recipient(s) or use the WHOIS tool of The

Ultimate Troubleshooter. If all the recipient email

addresses check out as regards the domain part of the

email addresses, then one of the servers on the way to the recipient(s) has DNS problems – usually this will be one of the first 2 servers in the chain, your own mail server (or your network) or your ISP’s mail server.

Examples of typical SMTP error 512 messages : “5.1.2 -

Bad destination host 'DNS Hard Error looking up domain”,

or “SMTP Error 550 5.1.2 Host unknown – host cannot be

found”, or how about this fantastically informative error

message “5.1.2 The message could not be delivered

because the recipient's destination email system is

unknown or invalid. Please check the address and try

again, or contact your system administrator to verify

connectivity to the email system of the recipient.”.



In summary : most SMTP error 512 conditions are

caused by misspellings of the domain name part of a

recipient email address. However, with the proliferation

of spam, error 512 is also often encountered by automatic

“out-of-office” replies to junk mail because the domain

names used by junk mail are often bogus domain names.


513 or 5.1.3 – Address type is

incorrect (most mail servers)


Relaying denied or


required (a small

percentage of mail



SMTP Error 513 : This status code (from the sender’s

mail server) is usually symptomatic, in an Exchange +

Outlook environment, of the user’s Outlook Contacts

having been imported from another system or PST and

where some of the addresses are not defined correctly.

Or, in any environment it is simply that the end-user simply did enter the email address completely wrongly, such as copying it from a website and not replacing “at” with “@”, e.g. : (which should have been, or” (“, quotes, is not allowed in email addresses and is often

included in error as a result of copying and pasting an

email from somewhere).


The user should check all the recipient addresses in the

email, including those that were inserted from Contacts.

Note : the SMTP reply code 5.1.3 is often a secondary

reply code. Some mail servers, for example, might reply

“SMTP error 501 5.1.3 Invalid address”, or “SMTP error

553 5.1.3 User address required !”, or “SMTP error 501

5.1.3 Bad recipient address syntax”, or “SMTP error 513

Relaying Denied - Can not send e-mails to some

addresses”, or this excellently informational Exchange

Server 2007 error (the whole error message is in green

below) :

”SMTP error 550 5.1.3 STOREDRV.Submit; invalid

recipient address.


Delivery has failed to these recipients or distribution lists:

The format of the recipient's e-mail address isn't valid. A

valid address looks like this:

Microsoft Exchange will not try to redeliver this message

for you. Please check the e-mail address and try sending

the message again, or provide the following diagnostic text to your system administrator.


5.1.3: The format of the recipient e-mail address is not

valid. Valid SMTP e-mail addresses can contain only

letters, numbers, hyphens, periods, and only one @

symbol. Troubleshooting: Verify that the SMTP address of

the recipient is formatted correctly and resend the



SMTP Error 513 is also used by a small percentage of

mail servers to indicate a completely different error, namely that you need to authenticate to the mail server before being able to send your message (SMTP authentication).


A typical error message might be : “SMTP error 553

Authentication is required to send mail as”.

In such cases you simply need to configure your mail

server, or your email program to send emails with SMTP



523 or 5.2.3 -

The Recipient’s

mailbox cannot

receive messages

this big


SMTP Status 523 : This error will be received when the

total size of the message you have sent (ie: message + all

of its attachments) exceeds the size limits on the

Recipient’s server. Many companies implement the good

practice of configuring their servers with limits on the size

of emails they can receive to prevent their systems running out of space as a result of a spam attack where the spam emails contain large attachments, or as a result of valid but not very technically savvy senders sending enormous scans (through not knowing that scanning at 1200dpi rather than the usually perfectly usable and acceptable 300dpi, will create humongous attachments).

Check the size of the email you sent, and, specifically, the

size of the attachments you included, and consider splitting your email into smaller emails. If that does not work, check with the Recipient the maximum size of email they can receive, and if that is still prohibitive then consider FTP arrangements between you and the recipient.


SMTP Error 523 is often a secondary SMTP error code

rather than a primary error code, as in the following

examples : “SMTP Error 450 5.2.3 Message Size greater

than allowed by Remote Host” or “SMTP Error 552 5.2.3

Data size exceeds maximum permitted” or “SMTP Error

552 5.2.3 Message exceeds maximum fixed size”, and so

on ....


550 or 5.5.0 – Requested actions

not taken as the

mailbox is



SMTP Error 550 : This response can be caused by quite

a few situations.


SMTP Error 550 will be returned by many servers If the

recipient email address simply does not exist on the

remote side (you will often get “550 Invalid recipient” or

“550 User account is unavailable” or “<ip-address-ofremote-

server> does not like recipient - 550 Address

rejected” or “550 No such user here” or “550 Not our

Customer” or “550 Account not available” or “Remote

host said : 550 –, this

THISCOMPANY.COM Mailbox Does Not Exist – Giving

up”). In this case the sender of the email needs to contact

the recipient verbally to get the correct email address.


SMTP Error 550 will sometimes also be returned by the recipient’s anti-spam firewall if, for example, the

anti-spam firewall does not like the sender (typically

because the sender needs to be whitelisted). A typical

example of an SMTP Error 550 return message by an

anti-spam firewall might be : does not like recipient.

Remote host said: 550-Verification failed for

550-Previous (cached) callout verification failure

550 Sender verify failed

Giving up on


SMTP Error 550 will also be returned if the user’s

mailbox is not local and Mail Relay is not enabled, or

the sending address is invalid (the latter is a way, by the remote server, to control spam).


Other situations of SMTP Error 550 include sending mail

to recipients outside of your domain where this is not



SMTP Error 550 is also returned when you are attempting

to send through a server which requires SMTP

authentication and you have not supplied credentials (ie.

your mail server, or email program, is attempting to send

without SMTP authentication)


Yet another set of circumstances where an SMTP error

550 might be issued include an incorrect From address

when used with an ISP where you can send mail only if

the From address is from a domain that they host for

you (at the time of writing, September 2008, British

Telecom in the UK is such an ISP – you have to notify

them through a lengthy, ridiculous, and almost soul

destroying procedure, involving proving that you own the

domain, for them to allow you to send emails from a

domain name that they do not host for you).


Another case of SMTP Error 550 is when the recipient’s

server is down (or cannot receive mail at this time) and

the ISP’s servers will retry periodically for a limited amount of time (this is often accompanied by a return mail from your ISP informing the sender of the email of just that situation).


Another case of SMTP Error 550 is when the recipient’s

server requires you to make a change to the To part of

your email to achieve successful delivery of the email

(some organizations configure their receiving mail servers

in this way when they have changed their domain name

and want to force the senders to update his address books – for example, has changed its domain to and you are still using the old domain name).


Yet another set of circumstances when the SMTP Error

550 is received is when the recipient’s mailbox has

been suspended. For instance, the QMAIL SMTP mail

program has an endearing way of telling you about a

mailbox that has been suspended : “I'm afraid I wasn't

able to deliver your message to the following addresses.

This is a permanent error; I've given up. Sorry it didn't work out. <email-address@email-domain>: <ip-address-ofremote-server> does not like recipient. Remote host said: 550 [SUSPEND] Mailbox currently suspended – Please contact correspondent directly.”.


Another circumstance of an SMTP Error 550 is when the

recipient’s mailbox has been disabled. The typical

reasons for this are the mailbox being full (the user needs

to delete messages before new ones will be accepted) or

the user not having paid a bill. An example of the reply

you will receive is : “550 mailbox temporarily disabled”


551 or 5.5.1 – User not local or

invalid address –

Relay denied.


SMTP Error 551 : If neither the sending mail address nor

the recipient’s address are locally hosted by the server,

then the ISP’s servers may refuse to relay the message

on. Some ISPs implement this restriction to thwart spammers. In our view, here at AnswersThatWork, this is a lazy and incompetent method of fighting spam as most of the time it does nothing but inconvenience no-one other than the ISP’s vast majority of considerate and law abiding users. In our experience this usually goes hand in hand with barely competent technical support. At the time of writing, 14-Sep-2008, a typical culprit for this is BT, British Telecom, in the UK. The way in which it manifests itself is as follows : you have a domain that is hosted by but your ISP is and

you try to send emails from your domain to


Neither your domain nor Yahoo.usa are hosted by, as a result your email is not accepted by

DodgyISP’s mail servers and your mail server is returned

an SMTP Error 551. To correct the problem you have to

call and ask them to enter your domain

name as an allowed sender.


552 or 5.5.2 – Requested mail

actions aborted –

Exceeded storage



SMTP Error 552 : The recipient’s mailbox has reached its

maximum allowed size (this is often accompanied by a

return mail from your ISP or mail server informing the

sender of the email of just that situation).

Example : “552 sorry, mailbox

is over quota temporarily (#5.1.1)”.


Some mail servers have extended the scope of SMTP

Reply Code 552 by also including errors where the size of the incoming message exceeds the size limit specified by the Network Administrator, as in, for example, “SMTP

Error 552 5.2.3 Message size exceeds fixed maximum

message size (7000000)”, which effectively says that the

incoming message was larger than the 7MB limit

(7,000,000 bytes) set by the Network Administrator of the

recipient’s mail server.


553 or 5.5.3 – Requested action not

taken – Mailbox name



SMTP Error 553 : There is an invalid email address in the

“To“, “CC”, or “BCC” field of the email message.

Here is a typical SMTP Error 553 response :

”Hi. This is the QMAIL-send program at <ip-address>. I'm

afraid I wasn't able to deliver your message to the following addresses. This is a permanent error; I've given up. Sorry it didn't work out. <Email-address-you’re-sending-to> : <remote-mail-server-ip> does not like recipient. Remote host said: 553 5.3.0 <Email-address-you’re-sending-to>.

Addressee unknown. Giving up.”.


SMTP Status 553 is also sometimes returned by an ISP

mail server. When this happens this is almost always

because you are trying to send through a specific ISP’s

SMTP server and yet you are not connected to the Internet through that ISP, e.g. you are connected to the Internet through a Comcast broadband connection but your email program (Outlook Express, dialog boxs Mail, ...) is configured to send emails through the SMTP server of Tiscali. A typical such error message might be : “553 sorry, relaying denied from your location”.


554 or 5.5.4 – Transaction failed.

Nowadays SMTP

status 554 is in most cases returned when

the recipient server believes your email is

spam or your IP

address or ISP server has been blacklisted

on one or more Internet blacklists.


With Yahoo, on the other hand, this usually means the

email address does not exist or has been



With IBM’s Lotus

Domino this is either a Domino bug or a Disk Full error


SMTP Error 554 : There was a permanent error trying to

complete the mail transaction which will not be resolved by resending the message in its current form. Some change to the message and/or destination must be made for successful delivery.


For instance, Yahoo often returns the following if the

recipient email address does not exist on the Yahoo

systems : “554 delivery error: This user doesn't have a account”. Another typical Yahoo SMTP Error

554 reply is : “554 delivery error: Sorry your message to <Email-Address> cannot be delivered. This account has

been disabled or discontinued”.


In the case of an IBM Lotus Domino server on the other

hand, this is either a disk full error (the first thing to

check), or a Lotus Domino bug which has appeared in

various guises in many versions of Lotus Domino as far

back as 2002. Eight years, Eight years and counting and

IBM still suffer from this bug – the mind boggles! Click

on this link to read the latest IBM Knowledgebase article

on this error at the time of writing :


In most other cases, however, a recipient mail server will

return an SMTP REPLY 554 when its anti-spam firewall

does not like the sender’s email address, or the

sender’s IP address, or the sender’s ISP server

(because, for example, they are listed in an RBL) and

where you will therefore either need to have the sender

whitelist you in their anti-spam program/appliance, or,

worse, you will need to take steps to have either your IP

address or your ISP’s servers (if you send mail through

your ISP) de-listed from one or more RBLs (RBL =

Realtime Blackhole List – also called Realtime Blacklist



For example, a 554 error returned by a Comcast server

might look like this : “ SMTP error from remote mail server after initial connection : host :

554 <Your-server-

IP-address> was found on one or more DNSBLs, see, where

DNSBLs = DNS Blacklists. In this case, therefore, if you

get such a message back it is telling you your IP address,

or your ISP’s mail server is listed on one of the anti-spam

blacklist databases that Comcast uses to filter out spam on incoming emails to Comcast mailboxes – click the link

provided in the error message to see how you may be able to un-blacklist yourself as far as Comcast is concerned. Here is another example from the OZEMAIL ISP in Australia, “SMTP error from remote mail server after initial connection to host :

554” – not very informative, as you

can see, but the name of the server returning the SMTP

reply 554 is what gives this away as OZEMAIL’s anti-spam not liking you : “Filter” in the name of a recipient server is almost always an indication that that server is an anti-spam and antivirus server. And here is a Twitter example : “SMTP error: Email Error: RCPT TO invalid mail server response:

554 5.7.1 : Recipient address rejected”.


554  or 5.5.4 – Transaction failed.

Nowadays SMTP

status 554 is in most

cases returned when

the recipient server

believes your email is

spam or your IP

address or ISP server

has been blacklisted

on one or more Internet blacklists.


With Yahoo, on the

other hand, this

usually means the

email address does

not exist or has been



With IBM’s Lotus

Domino this is either

a Domino bug or a

Disk Full error


Note that SMTP Error 554 can also often be buried in the

middle of SMTP Error 550 errors. Here is an example of a

recipient mail server returning an SMTP Error 554 because its Barracuda anti-spam firewall appliance rejected the email (the cause, as shown below, is Barracuda Reputation which means your IP address or your ISP’s server is blacklisted on Barracuda’s RBL) : does not like recipient.

Remote host said: 550-Verification failed for


550-Sent: RCPT


554 : Service unavailable; Client host

[] blocked using Barracuda



550 Sender verify failed

Giving up on


The following addresses had

permanent delivery errors


“The following addresses had permanent delivery

errors” / “The following address had permanent

delivery errors” : Either of these sentences are usually

followed by one or more email address(es).

The error message is effectively saying that the email

addresses listed do not exist, or no longer exist (if you

used to be able to email to them successfully). You need

to get the sender to verbally verify with the recipient what

his/her new email address is.