Reaching the Inbox, Part 4 – SMTP Error Codes

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See also:

Reaching the Inbox, Part 1 – Why not all emails get delivered

Reaching the Inbox, Part 2 – Factors that impact email deliverability

Reaching the Inbox, Part 3 – Understanding bounce-backs

SMTP Error Codes

For every email you send, the receiving email server will return a 3-digit SMTP code to your sending server.

The first digit defines whether the server has accepted the command, completed an action, identified a temporary issue or encountered an error. The second and third digits provide further information about syntax, connection status, mail transfer status etc.

The key ones to look out for in bounce backs are 5xx codes, especially the codes from 550 to 555.

Unfortunately, as mentioned previously, the situation gets rather confusing because:

  • not all email servers use the standard SMTP bounce reply codes in the same way, meaning your ESP must guess how to categorize that bounced message
  • not all ESPs use the same rules when categorising bounce-backs, meaning one sender might get a hard bounce, while another sender using a different ESP gets a soft bounce from the same email address

On the plus side, many error codes come with accompanying text to help explain the reason for the error.

Examples of side messages for code 550, the most common SMTP error code meaning simply the email could not be delivered, include:

550 Message rejected as spam

550 Mail from refused spam site

550 Unrouteable address

550 Message contained unsafe content


101 The server is unable to connect. Try to change the server’s name (maybe it was spelt incorrectly) or the connection port.
111 Connection refused or inability to open an SMTP stream. This error normally refers to a connection issue with the remote SMTP server, depending on firewalls or misspelled domains. Double-check all the configurations and in case ask your provider.
211 System status message or help reply. It comes with more information about the server.
214 A response to the HELP command. It contains information about your particular server, normally pointing to a FAQ page.
220 The server is ready. It’s just a welcome message. Just read it and be happy that everything is working (so far)!
221 The server is closing its transmission channel. It can come with side messages like “Goodbye” or “Closing connection”. The mailing session is going to end, which simply means that all messages have been processed.
250 Its typical side message is “Requested mail action okay completed”: meaning that the server has transmitted a message. The opposite of an error: everything has worked and your email has been delivered.
251 “User not local will forward”: the recipient’s account is not on the present server, so it will be relayed to another. It’s a normal transfer action. For other information check out our article on what is an SMTP server.
252 The server cannot verify the user, but it will try to deliver the message anyway. The recipient’s email account is valid, but not verifiable. Normally the server relays the message to another one that will be able to check it.
354 The side message can be very cryptic (“Start mail input end <CRLF>.<CRLF>”). It’s the typical response to the DATA command. The server has received the “From” and “To” details of the email and is ready to get the body message.
420 “Timeout connection problem”: there have been issues during the message transfer. This error message is produced only by GroupWise servers. Either your email has been blocked by the recipient’s firewall, or there’s a hardware problem. Check with your provider.
421 The service is unavailable due to a connection problem: it may refer to an exceeded limit of simultaneous connections, or a more general temporary problem. The server (yours or the recipient’s) is not available at the moment, so the dispatch will be tried again later.
422 The recipient’s mailbox has exceeded its storage limit. Best is to contact the user via another channel to alert him and ask to create some free room in his mailbox.
431 Not enough space on the disk, or an “out of memory” condition due to a file overload. This error may depend on too many messages sent to a particular domain. You should try again sending smaller sets of emails instead of one big mail-out.
432 Typical side-message: “The recipient’s Exchange Server incoming mail queue has been stopped”. It’s a Microsoft Exchange Server’s SMTP error code. You should contact it to get more information: generally it’s due to a connection problem.
441 The recipient’s server is not responding. There’s an issue with the user’s incoming server: yours will try again to contact it.
442 The connection was dropped during the transmission. A typical network connection problem, probably due to your router: check it immediately.
446 The maximum hop count was exceeded for the message: an internal loop has occurred. Ask your SMTP provider to verify what has happened.
447 Your outgoing message timed out because of issues concerning the incoming server. This happens generally when you exceeded your server’s limit of number of recipients for a message. Try to send it again segmenting the list in different parts.
449 A routing error. Like error 432, it’s related only to Microsoft Exchange. Use WinRoute.
450 “Requested action not taken – The user’s mailbox is unavailable”. The mailbox has been corrupted, placed on an offline server, or your email hasn’t been accepted for IP problems or blacklisting. After some time, the server will try to resend the message. Some ESPs use shared IP addresses for multiple clients, so you should also check the status of your sending IP address.
451 “Requested action aborted – Local error in processing”. Your ISP’s server or the server that got a first relay from yours has encountered a connection problem. It’s normally a transient error due to a message overload, but it can refer also to a rejection due to a remote antispam filter. If it keeps repeating, ask your SMTP provider to check the situation. (If you’re sending a large bulk email with a free one that can be a common issue).
452 Too many emails sent or too many recipients: more in general, a server storage limit exceeded. Again, the typical cause is a message overload. Usually the next try will succeed: in case of problems on your server it will come with a side-message like “Out of memory”.
471 An error of your mail server, often due to an issue of the local anti-spam filter. Contact your SMTP service provider to fix the situation.
500 A syntax error: the server couldn’t recognize the command. It may be caused by a bad interaction of the server with your firewall or antivirus. Read carefully their instructions to solve it.
501 A syntax error in the parameters or arguments of the command. Commonly, this is due to an invalid email address, but it can also be associated with connection problems, or an issue concerning antivirus settings.
502 The command is not implemented. The command has not been activated by your own server. Contact your provider to know more about it.
503 The server has encountered a bad sequence of commands, or it requires an authentication. In case of “bad sequence”, the server has pulled off its commands in a wrong order, usually because of a broken connection. If an authentication is needed, you should enter your username and password.
504 A syntax error: a command parameter is not implemented. Like error 501, is a syntax problem; you should ask your provider.
510/511 Bad email address. One of the addresses in your TO, CC or BCC line doesn’t exist. Check again your recipients’ accounts and correct any possible misspelling.
512 A DNS error: the host server for the recipient’s domain name cannot be found. Check again all your recipients’ addresses: there will likely be an error in a domain name (like [email protected] instead of [email protected]).
513 “Address type is incorrect”: usually a problem concerning address misspelling. In some cases, however, it’s related to an authentication issue. Check the recipients’ addresses and correct any mistake. If everything’s ok and the error persists, then it’s caused by a configuration issue (the server needs an authentication).
523 The total size of your mailing exceeds the recipient server’s limits. Re-send your message splitting the list in smaller subsets.
530 Typically, an authentication problem, but sometimes it may be the recipient’s server blacklisting yours, or an invalid email address. Configure your settings providing a username+password authentication. If the error persists, check all your recipients’ addresses and whether you have been blacklisted.
541 The recipient address rejected your message: normally, it’s an error caused by an anti-spam filter. Your message has been detected and labeled as spam. You must ask the recipient to whitelist you.
550 Usually caused by a non-existent email address. However, this code can sometimes be returned by the recipient’s firewall (or when the incoming server is down). The majority of 550 errors occur because the recipient email address does not exist, in which case you should contact the recipient to get the right address.
551 “User not local or invalid address – Relay denied”. If both your address and the recipient’s are not locally hosted by the server, a relay can be interrupted. It’s a (not very clever) strategy to prevent spamming. You should contact your ISP and ask them to allow you as a certified sender. You are unlikely to come across this issue if you use a professional SMTP provider.
552 “Requested mail actions aborted – Exceeded storage allocation”. This indicates the recipient’s mailbox has exceeded its limits. This typically occurs when sending emails with big attachments. Try re-sending with a smaller attachment.
553 “Requested action not taken – Mailbox name invalid”. Check all the addresses in the TO, CC and BCC fields. There will likely be an error or a misspelling somewhere.
554 This means that the transaction has failed. It’s a permanent error and the server will not try to send the message again. The incoming server thinks your email is spam, or your IP has been blacklisted. Check carefully if you ended up in some spam lists.

Meet The Author

Kevin Savage

Former VP of Sales & Marketing at Yankee Group with research expertise for the technology sector Previously: Successfully growing new data and analytics markets for CEB/Gartner as well as Communispace, Jupiter Research, Forrester Research.

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