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Outgoing SMTP errors

What are SMTP errors?

When you send an email from an email client(like Microsoft outlook, mac mail, Mozilla thunderbird etc) or webmail interface(email interface the hosting provider provides you) and the email is not delivered to the person you are sending the email to, you would get an email back. This email you get back is called a bounce message.

In this bounce message, it will say your email has not been delivered due to a specific reason. It gives you an explanation why the email was not delivered and it would give you an error code(for instance 550, 550, 553, 554 etc) together with the email headers of the email you sent.

How to troubleshoot SMTP errors

Here are a few error codes you would get in a bounce message, the explanation what they mean and how you can troubleshoot and resolve the email sending issues:

Code: 101
Meaning: The server is unable to connect.
How to solve the issue: Try to change the server’s name (maybe it was spelled incorrectly) or the connection port.

Code: 111
Meaning: 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.

Code: 211
Meaning: System status message or help reply.
How to solve the issue: It comes with more information about the server.

Code: 214
Meaning: A response to the HELP command.
How to solve the issue: It contains information about your particular server, normally pointing to a FAQ page.

Code: 220
Meaning: The server is ready.
How to solve the issue: It’s just a welcome message. Just read it and be happy that everything is working (so far)!

Code: 221
Meaning: The server is closing its transmission channel. It can come with side messages like “Goodbye” or “Closing connection”.
How to solve the issue: The mailing session is going to end, which simply means that all messages have been processed.

Code: 250
Meaning: Its typical side message is “Requested mail action okay completed”: meaning that the server has transmitted a message.
How to solve the issue: The opposite of an error: everything has worked and your email has been delivered.

Code: 251
Meaning: “User not local will forward”: the recipient’s account is not on the present server, so it will be relayed to another.
How to solve the issue: It’s a normal transfer action. For other information check out our article on what is an SMTP server.

Code: 252
Meaning: The server cannot verify the user, but it will try to deliver the message anyway.
How to solve the issue: 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.

Code: 354
Meaning: The side message can be very cryptic (“Start mail input end <CRLF>.<CRLF>”). It’s the typical response to the DATA command.
How to solve the issue: The server has received the “From” and “To” details of the email, and is ready to get the body message.

Code: 420
Meaning: “Timeout connection problem”: there have been issues during the message transfer.
How to solve the issue: 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.

Code: 421
Meaning: 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.
How to solve the issue: The server (yours or the recipient) is not available at the moment, so the dispatch will be tried again later.

Code: 422
Meaning: The recipient’s mailbox has exceeded its storage limit.
How to solve the issue: Best is to contact the user via another channel to alert him and ask to create some free room in his mailbox.

Code: 431
Meaning: Not enough space on the disk, or an “out of memory” condition due to a file overload.
How to solve the issue: 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.

Code: 432
Meaning: Typical side-message: “The recipient’s Exchange Server incoming mail queue has been stopped”.
How to solve the issue: 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.

Code: 441
Meaning: The recipient’s server is not responding.
How to solve the issue: There’s an issue with the user’s incoming server: yours will try again to contact it.

Code: 442
Meaning: The connection was dropped during the transmission.
How to solve the issue: A typical network connection problem, probably due to your router: check it immediately.

Code: 446
Meaning: The maximum hop count was exceeded for the message: an internal loop has occurred.
How to solve the issue: Ask your SMTP provider to verify what has happened.

Code: 447
Meaning: Your outgoing message timed out because of issues concerning the incoming server.
How to solve the issue: 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.

Code: 449
Meaning: A routing error.
How to solve the issue: Like error 432, it’s related only to Microsoft Exchange. Please contact Microsoft exchange support to make sure all the DNS settings are correct and if they as us to implement something on your domain, please don’t hesitate the as us to help you adjust the DNS settings on your domain.

Code: 450
Meaning: “Requested action not taken – The user’s mailbox is unavailable”. The mailbox has been corrupted or placed on an offline server, or your email hasn’t been accepted for IP problems or blacklisting.
How to solve the issue: The server will retry to mail the message again, after some time. Anyway, verify that is working with a reliable IP address.

Code: 451
Meaning: “Requested action aborted – Local error in processing”. Your ISP’s server or the server that got the first relay from yours has encountered a connection problem.
How to solve the issue: 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).

Code: 452
Meaning: Too many emails sent or too many recipients: more in general, a server storage limit exceeded.
How to solve the issue: 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”.

Code: 471
Meaning: An error of your mail server, often due to an issue of the local anti-spam filter.
How to solve the issue: Contact your SMTP service provider to fix the situation.

Code: 500
Meaning: A syntax error: the server couldn’t recognize the command.
How to solve the issue: It may be caused by a bad interaction of the server with your firewall or antivirus. Read carefully their instructions to solve it.

Code: 501
Meaning: Another syntax error, not in the command but in its parameters or arguments.
How to solve the issue: In the majority of the times it’s due to an invalid email address, but it can also be associated with connection problems (and again, an issue concerning your antivirus settings).

Code: 502
Meaning: The command is not implemented.
How to solve the issue: The command has not been activated yet on your own server. Contact your provider to know more about it.

Code: 503
Meaning: The server has encountered a bad sequence of commands, or it requires an authentication.
How to solve the issue: 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.

Code: 504
Meaning: A command parameter is not implemented.
How to solve the issue: Like error 501, is a syntax problem; you should ask your provider.

Code: 510/511
Meaning: Bad email address.
How to solve the issue: One of the addresses in your TO, CC or BBC line doesn’t exist. Check again your recipients’ accounts and correct any possible misspelling.

Code: 512
Meaning: A DNS error: the host server for the recipient’s domain name cannot be found.
How to solve the issue: Check again all your recipients’ addresses: there will likely be an error in a domain name (like instead of

Code: 513
Meaning: “Address type is incorrect”: another problem concerning address misspelling. In few cases, however, it’s related to an authentication issue.
How to solve the issue: Double-check your recipients’ addresses and correct any mistake. If everything’s ok and the error persists, then it’s caused by a configuration issue (simply, the server needs an authentication).

Code: 523
Meaning: The total size of your mailing exceeds the recipient server’s limits.
How to solve the issue: Re-send your message splitting the list in smaller subsets.

Code: 530
Meaning: Normally, an authentication problem. But sometimes it’s about the recipient’s server blacklisting yours, or an invalid email address.
How to solve the issue: Configure your settings providing a username and password authentication. If the error persists, check all your recipients’ addresses and if you’ve been blacklisted.

Code: 541
Meaning: The recipient address rejected your message: normally, it’s an error caused by an anti-spam filter.
How to solve the issue: Your message has been detected and labeled as spam. You must ask the recipient to whitelist you.

Code: 550
Meaning: It usually defines a non-existent email address on the remote side.
How to solve the issue: Though it can be returned also by the recipient’s firewall (or when the incoming server is down), the great majority of errors 550 simply tell that the recipient email address doesn’t exist. You should contact the recipient otherwise and get the right address.

Code: 551
Meaning: Relay request denied. Meaning, the address you are sending from is hosted by us and it is not authenticating correctly through the email relay provider.
How to solve the issue: You should contact us and ask us to double check if your domain is correctly implemented on our outgoing email filtering relay system.

Code: 552
Meaning: “Requested mail actions aborted – Exceeded storage allocation”: simply put, the recipient’s mailbox has exceeded its limits.
How to solve the issue: Try to send a lighter message: that usually happens when you dispatch emails with big attachments, so check them first. Or you can contact the person you are sending the email to and tell them they have reached their email mailbox quota, they should free up some space on their email account on the email server or they should contact their provider for assistance.

Code: 553
Meaning: “Requested action not taken – Mailbox name invalid”. That is, there’s an incorrect email address into the recipients’ line.
How to solve the issue: Check all the addresses in the TO, CC and BCC field. There should be an error or a misspelling somewhere.

Code: 554
Meaning: This means that the transaction has failed. It’s a permanent error and the server will not try to send the message again.
How to solve the issue: The incoming server thinks that your email is spam, or your IP has been blacklisted. Check carefully if you ended up on some spam lists. Please check for any blacklisting issues on your domain.

If you struggle to troubleshoot any of the above errors, please feel free to log into your Customer Zone and send us a support ticket so we can assist further.

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