What Are SMTP Error Codes And Why You Should Care About It?

SMTP Error Code

As an email marketer, you must have gone through the point where your email marketing campaign fails and you receives a message something like “SMTP Error Code” with a meaningless message. Uuff!! The most dreadful moment. Isn’t it?

So what is SMTP?

SMTP is an acronym for Simple Mail Transfer Protocol. It is an internet standard for electronic mail and is widely used today. SMTP is a protocol which is responsible for sending and receiving emails. Multiple processes communicate with each other through SMTP,  just like SalesPush functions as SMTP service to send billions of marketing and transactional emails every month.

Then what is SMTP Error Message?

All emails sent by a client, mobile device or server use SMTP to deliver the messages from source to destination mail server. Once the message has been received by the mail server, then the user collects that message using a protocol called POP3 (Post Office Protocol) or IMAP (Internet Message Access Protocol).

Now, every time an email is sent using SMTP, the receiving server will respond with an SMTP error code. Not all SMTP codes indicate failures.

SMTP codes generally consist of three different digits that carry different meanings.
Some of the common SMTP codes are mentioned below with their meanings:

The server is unable to connect.
Try to change the server’s name (maybe it was spelled incorrectly)
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.
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 an FAQ page.
The server is ready.
It’s just a welcome message.
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”
Everything has worked and your email has been delivered.
“User not local will forward”
It’s a normal transfer action.
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.
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”
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
The server (yours or the recipients) 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 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.
“The recipient’s Exchange Server incoming mail queue has been stopped”.
It’s a Microsoft Exchange Server’s SMTP error code. 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 a 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 the 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 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.

A command parameter is not implemented.
Like error 501, is a syntax problem; you should ask your provider.
Bad email address.
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 ([email protected] instead of [email protected]).
“Address type is incorrect”: another problem concerning address misspelling. In a few cases, however, it’s related to an authentication 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).
The total size of your mailing exceeds the recipient server’s limits.
Re-send your message splitting the list into 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 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 action 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 recipient’s 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.

Why you should care about it?

As an email marketer, this is the most dreadful phase and one must know how to tackle such situations with ease. Email marketer must know what, when and how happened behind these SMTP code responses.


Have you faced any other SMTP error code?
If yes, please share with us in the comment section below.


Please enter your comment!
Please enter your name here