SMTP Error Messages

SMTP Server Issues

In the following article we will describe in detail the response and error codes related to SMTP servers, in order to allow you to remedy any inconvenience.

When there are problems with the delivery of an email (for example after a bounce), your SMTP server issues a particular error message. Unfortunately this is usually a very cryptic communication, such as “550 Requested action not taken: mailbox unavailable” or “421 Try again later”. What do these numbers mean?

Firstly: not all codes issued by the SMTP server are errors. Sometimes these are just responses to a command, containing a detail of the operation. More specifically, each code consists of three digits that convey particular information: the first clarifies whether the server has accepted the command, performed an action, encountered an error and so on; while the second and third specify the description by indicating whether it is a syntactic problem, a connection problem, an unspecified one, and so on.

Unfortunately, different servers often use these codes in different ways, making the whole matter even more complicated … In any case, the most critical series of error messages is the one that begins with the digit 5, or the 5xx series – and in particular the one that goes from 550 to 559. The most common error of all is however the code 550: that is, a problem concerning the address of the recipient (generally non-existent).

That said, here is a list of the main SMTP errors or response codes with an explanation and advice on what to do.

CODE	MEANING	HOW TO BEHAVE
101	The server cannot connect.	Try changing the server name (maybe it was misspelled) or the connection port.
111	Connection refused or inability to open the SMTP stream.	This kind of error is typically related to a connection problem with the remote SMTP server, caused by a blockage in the firewall or by incorrectly written domains. Check all settings and contact your provider if necessary.
211	Status message.	More information is usually given about it, but it is not a mistake.
214	Response to the HELP command.	It contains information related to your server, and usually points to an FAQ page.
220	The server is ready.	It is simply a welcome message: everything is fine!
221	The server is closing its transmission channel.	The sending session is about to end, which simply means that all messages have been processed.
250	The code is usually accompanied by the message “Requested mail action okay completed”.	The exact opposite of a mistake: everything went perfectly and your email was delivered.
251	The message alongside is “User not local will forward”: the recipient’s account is not on the current server, so it will be relayed to another.	This is a typical transfer action; no problem.
252	The server cannot verify the user, but will try to deliver the message anyway.	The recipient’s email is valid, but not verifiable: typically the server transfers the message to another host capable of verifying.
354	The explanatory message can be very cryptic (“Start mail input end ”). In any case, it is the response to the DATA command.	The server has received the “From” and “To” details of the email (sender and recipient) and is ready to receive the body of the message.
420	“Timeout connection problem”: there were some interruptions during the delivery.	This error is produced only by GroupWise servers; contact your provider immediately.
421	The service is unavailable due to a connection problem: it can refer to time limits that have been exceeded due to simultaneous connections, or to another more general problem.	The server (yours or the recipient’s) is currently unavailable, so delivery will be delayed. However, this is a temporary difficulty.
422	The recipient’s mailbox has exceeded the space limits.	It is best to contact the user via another communication channel and notify them that their inbox is full.
431	There is not enough disk space or there has been an overloaded file.	The cause may be that too many messages are being sent to a particular domain. You should try to send smaller segments of emails instead of one large mailing.
432	The typical side-message is: “The recipient’s Exchange Server incoming mail queue has been stopped”.	This is a Microsoft Exchange error code. You should contact Microsoft customer support for more information; however, it is usually a connection problem.
441	The recipient’s server is not responding.	A temporary failure of the incoming mail server; yours will try to contact him again.
442	The connection was dropped during transmission.	Typical network error, probably due to your router: check it now.
446	An “internal loop” has been created.	Contact your SMTP provider’s customer service.
447	Your outgoing message has run out of time due to an incoming mail server problem.	This typically happens when you exceed the limit of emails that can be sent at one time allowed by your SMTP server. To solve it definitively you can always rely on a professional server .
449	A routing error.	Like error 432, it is related only to Microsoft Exchange. Use WinRoute .
450	“Requested action not taken – The user’s mailbox is unavailable”. The recipient’s mailbox has been compromised or placed on an offline server; or your email was not accepted due to IP problems.	The server will try to send the message again after some time. In any case, it is good to check that your IP address is reliable.
451	“Requested action aborted – Local error in processing”. Your ISP server had a connection problem.	This is usually a temporary error due to an overload of messages, but it can also be referred to an email rejection related to the spam filter. If it keeps repeating, contact your SMTP provider: if you are trying to send a mass email with a free server this is a common problem; in this case our advice is to opt for a professional service .
452	Too many emails or too many recipients: in general, limits exceeded.	Again, a typical overload case. The next attempt should usually work.
471	An error on your mail server, usually due to a spam filter problem.	Contact your SMTP service to resolve the situation.
500	A syntax error: the server could not recognize the command.	It can be caused by the server not interacting with your firewall or antivirus. Consult the instructions for these programs.
503	The server has encountered a “bad sequence of commands”, or requires authentication.	In the case of the “bad sequence”, the server generated commands in the wrong order, basically due to an interruption in the connection. If, on the other hand, it is required authentication, you should enter your username and password in the configuration window.
504	A command parameter is not implemented.	Like error 501, it is a syntax problem: contact your provider.
510/511	Wrong email address.	One of the addresses on your A, CC or BCC lines does not exist. Check the recipient accounts again and correct any types.
512	A DNS error: The recipient’s domain host server could not be found.	Check all recipient addresses again – there will likely be an error in the domain name (such as [email protected] instead of [email protected]).
513	“Address type is incorrect”: another problem related to incorrectly written addresses. In some cases, however, it may be related to an authentication problem.	Check the addresses of the recipients and correct the errors: if everything is ok then the server needs authentication (easily configurable in the mail client settings).
523	The total weight of the mailing exceeds the limits of the incoming mail server.	Re-send your message by dividing the list into smaller segments.
530	Generally, it is an authentication problem: but sometimes it is related to the recipient’s server blacklisting yours, or to an error in the email addresses.	Configure your client with username + password authentication and double-check all the email addresses you send to find any errors. If the problem persists, check that you have not ended up on a blacklist.
541	The recipient’s address refused your essay. This is usually an error caused by a spam filter.	Your message has been tagged as spam; you must ask the recipient to remove this report.
550	Typically defines the presence of a non-existent email address to which you are trying to send a message.	While it can also be linked to firewall or server down issues, the vast majority of 550 errors simply report the non-existence of an address to which you are sending, usually due to a typo.
551	“User not local or invalid address – Relay denied”. If both your email address and the recipient’s are not on the local server, a relay can be broken.	This is a (not very smart) strategy for preventing spam. You should contact your ISP and ask them to certify you as a non-spammer. Of course, with a professional SMTP service this problem will never arise.
552	“Requested mail actions aborted – Exceeded storage allocation”: simply, the recipient’s mailbox has exceeded its space limits.	Try sending a lighter message: this error usually occurs when you send attachments that are too large.
553	“Requested action not taken – Mailbox name invalid”. ‘is an incorrect address on the recipient line.	Check all email addresses in the To, CC and BCC fields – there should be some typo.
554	The transaction failed. This is a permanent error and your server will no longer attempt to send the message.	Typically the recipient’s incoming mail server thinks your email is spam, or your IP has been blacklisted. Check that this is not the case, or rely on a professional service such as turbo SMTP which will radically eliminate the problem.