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Asked: 2016-02-23 07:59:29 +0800 CST

Explanation of various smtp error codes bounced by mailservers

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I'd like to ask real mailserver administrators from here about some smtp error codes - I take care of some customer's mailing systems, and there is a wide range of server replies, many of them not very clear and almost every server has it's own reply. If I get correct and clear reply like "unknown user", "invalid recipient", "blocked due content restrictions" etc, I can process it and do the required changes.

But I'm really confused about the replies containing "hop counts", "relaying", "access denied" without any additional info, smtp authorization required etc. I need to identify what the bounce replies really mean, if it is bad server reply od recipient side, error on sender side, technical problem, needed domain whitelist, etc. Here is a list of some replies with not really clear meaning for me:

we do not relay
relaying denied
smtp; 550 5.7.1 Unable to relay
Recipient address rejected
Recipient address rejected: Access denied

554 5.1.0 Sender denied
smtp; 550 relay not permitted
Relay access denied

Hop count exceeded - possible mail loop
Maximum forwarding loop count exceeded

554 5.7.1 Access denied
Recipient address rejected: Access denied
550 #5.1.0 Address rejected

RESOLVER.RST.AuthRequired; authentication required

550 Denied by policy
553 5.7.1 sorry, that domain isn't in my list of allowed

550 5.7.1 Command not allowed
smtp

1 Answers

  1. In general the SMTP RFC 2821 doesn't define error messages content, it only establish some basic rules on whet should be done and how it should be done. So most SMTP relay administrator usually write their own custom messages, in a way that is understandable to them. This is done primarily in order to being able to distinguish various bounce reasons in their own logs.

    It's probably not worth guessing what did every message mean exactly, but I'll try to explain the average meaning on these. Notice I regrouped the messages basing on their meaning.

    • we do not relay - a general message without explanation. Ask relay staff assistance.
    • relaying denied - same.
    • Relay access denied - same.
    • smtp; 550 5.7.1 Unable to relay - same.
    • 554 5.7.1 Access denied - same.
    • 550 Denied by policy - same.

    • smtp; 550 relay not permitted - probably same as above.

    • Recipient address rejected: Access denied - relay found that it doesn't want to relay the message to the e-mail specified in RCPT TO:, but didn't state the exact reason.

    • Recipient address rejected - same.

    • Recipient address rejected: Access denied - same.

    • 554 5.1.0 Sender denied - after you SMTP client issued MAIL FROM: the relay probably found this address in some sort of black list. Ask relay staff for assistance.

    • Hop count exceeded - possible mail loop - incorrectly configured DNS or relay - report to relay stuff.

    • Maximum forwarding loop count exceeded - same.

    • 550 #5.1.0 Address rejected - depends on what address is rejected, could be both MAIL FROM: or RCPT TO:. In either case the reason isn't stated.

    • RESOLVER.RST.AuthRequired; authentication required - relay wants SMTP AUTH.

    • 553 5.7.1 sorry, that domain isn't in my list of allowed - relay didn't recognize the domail part as local. Probably it's configured incorrectly, or it's a DNS issue. Report to remote relay staff.

    • 550 5.7.1 Command not allowed - incorrect SMTP command order, for example DATA after MAIL FROM:, missed HELO or similar.

    When asking for assistance you can probably start with wrinting a message describing a problem to the [email protected] e-mail address, but nowdays the availability of a person reading this mailbox, or even the presence of this e-mail on the destination relay cannot be guaranteed.

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