SMTP errors explained Print

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Outgoing SMTP errors explained

 What are SMTP errors?

 When you send an email from an email client (like Microsoft Outlook, Mac Mail, Mozilla Thunderbird, etc.) or a Webmail interface (email interface the hosting provider provides to you) and the email is not delivered to the person you are sending the email to, you will get an email back. This email is called a “bounce message”.

 In the bounce message it will state the reason why your email has not been delivered. It gives you an explanation why the email was not delivered and it would give you an error code (such as 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 could get in a bounce message, what they mean and how you can troubleshoot and resolve the issue:


Code: 101

Meaning: The server is unable to connect.

How to solve the issue: Try to change the server’s name (maybe it was spelt 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 misspelt domains. Double-check all the configurations. You can also ask your provider to assist.


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 an FAQ page.


Code: 220

Meaning: The server is ready.

How to solve the issue: This is just a welcome message.


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 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 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 a 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”. This means that 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. t 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 space in his or her 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. 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. Your server 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 when you exceeded your server’s limit 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.


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 because of IP problems or blacklisting.

How to solve the issue: The server will retry to mail the message again, after some time. You can verify if it 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 also refer to a rejection due to a remote anti-spam 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, this can be a common issue.


Code: 452

Meaning: Too many emails sent or too many recipients: 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 recognise the command.”

How to solve the issue: It may be caused by a bad interaction of the server with your firewall or antivirus. Carefully read their instructions to solve it.


Code: 501

Meaning: Another syntax error.

How to solve the issue: The majority of the time, 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 the 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 mail@domain.coom 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: Doublecheck 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 + 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: “User not local or invalid address – Relay denied”. Meaning, if both your address and the recipient’s are not locally hosted by the server, a relay can be interrupted.

How to solve the issue: 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. Of course, with a professional SMTP provider like turboSMTP you won’t ever deal with this issue.


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.


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 in some spam lists, or rely on a professional SMTP service like turboSMTP that will nullify this problem.


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