SMTP errors and reply codes

Sometimes your SMTP server may return a particular error message. The problem is that it will generally be very cryptic, like “550 Requested action not taken: mailbox unavailable” or “421 Try again later”. What does these numbers mean?

First of all: not any reply code is an error. Sometimes it’s just a response containing a detail about the server or an answer to a command. Secondly: any code consist of three digits, and each conveys a particular information. The first one defines whether the server has accepted the command, fulfilled an action, run into a temporary issue, encountered an error etc; the second and the third one refine the description further, stating if there’s been a syntactic problem, or a connection trouble etc.

Unfortunately, different servers sometimes use these codes in a different way, making the whole thing even more complicated… Anyhow, the most critical series of error messages is the 5xx one, and especially the ones from 550 to 559. In particular, you will probably get a lot of 550 SMTP error codes – that is, a problem that concerns the recipient’s email address.

Finally, remember that it’s much easier to deal with these error codes if you choose to rely on a professional SMTP server that will help you solve any issue. turboSMTP, for instance, comes with a 24/7 customer support: you can try it free and forget once for all these issues.

And here’s a list of the main SMTP error or reply messages, with an explanation and a tip about what to do.





The server is unable to connect.

Try to change the server’s name (maybe it was spelt incorrectly) or the connection port.


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.


System status message or help reply.

It comes with more information about the server.


A response to the HELP command.

It contains information about your particular server, normally pointing to a FAQ page.


The server is ready.

It’s just a welcome message. Just read it and be happy that everything is working (so far)!


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.


Its typical side message is “Requested mail action okay completed”: meaning that the server has transmitted a message.

The oppsite of an error: everything has worked and your email has been delivered.


“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.


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.


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.


“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.


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.


The recipient’s mailbox has exceeded its storage limit.

Best is to contact contact the user via another channel to alert him and ask to create some free room in his mailbox.


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.


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.


The recipient’s server is not responding.

There’s an issue with the user’s incoming server: yours will try again to contact it.


The connection was dropped during the transmission.

A typical network connection problem, probably due to your router: check it immediately.


The maximum hop count was exceeded for the message: an internal loop has occurred.

Ask your SMTP provider to verify what has happened.


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.


A routing error.

Like error 432, it’s related only to Microsoft Exchange. Use WinRoute.


“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.

The server will retry to mail the message again, after some time. Anyway, verify that is working on a reliable IP address.


“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).


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”.


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.


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.


Another syntax error, not in the command but in its parameters or arguments.

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).


The command is not implemented.

The command has not been activated yet on your own server. Contact your provider to know more about it.


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.


A command parameter is not implemented.

Like error 501, is a syntax problem; you should ask your provider.


Bad email address.

One of the addresses in your TO, CC or BBC line doesn’t exist. Check again your recipients’ accounts and correct any possible misspelling.


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 mail@domain.coom instead of


“Address type is incorrect”: another problem concerning address misspelling. In few cases, however, it’s related to an authentication 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).


The total size of your mailing exceeds the recipient server’s limits.

Re-send your message splitting the list in smaller subsets.


Normally, an authentication problem. But sometimes it’s about 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 if you’ve been blacklisted.


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.


It usually defines a non-existent email address on the remote side.

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.


“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.

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.


“Requested mail actions aborted – Exceeded storage allocation”: simply put, the recipient’s mailbox has exceeded its limits.

Try to send a lighter message: that usually happens when you dispatch emails with big attachments, so check them first.


“Requested action not taken – Mailbox name invalid”. That is, there’s an incorrect email address into the recipients line.

Check all the addresses in the TO, CC and BCC field. There should be an error or a misspelling somewhere.


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 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.