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Determining the cause of a bounced message
The "transcript" in the message provides the main clues -- sometimes obvious, sometimes obscure -- to what actually happened. Additional evidence can also often be found in the bounce's Subject: line, which often contains a useful excerpt from the transcript.

Here are some of the many cases that have passed through our Help Desk. Not all lines shown below may appear in a given incident and sometimes a mail server will add others,

Session-transcriptOther specificsExplanation

>>> RCPT To:<recipient's-address>
>>> 550 <recipient's-address>... User unknown
550 <recipient's-address>... User unknown
--Recipient on the To: line is misspelled, or no longer has an account at that address

550 <recipient's-address>...
   Host unknown (Name server:
   recipient's-host:   host not found)
--Recipient's host-name on the To: line is misspelled, or its name-server is down

>>> MAIL From:<sender's address>
>>> 550 Access denied
...followed by one of three reasons at the right --->
550 Sorry, no access for this ipSender is from a known SPAM IP-address
550 Sorry, no access for this domainSender is in a known SPAM domain
550 Sorry, no access for this userSender is a known SPAMmer

550 /directory-path/.forward: line 1:
(contents of line 1) ... Can't create output:
Permission denied
--The recipient has a .forward file containing an unreadable address

>>> DATA
>>> 553 user address required
> 554 <recipient's-address>... Service unavailable


451 From:domain must resolve
--To: or From: line has an improperly constructed address.

553 <recipient's-address>
   ... unauthorized relay traffic prohibited
554 <recipient's-address>
   ... Service unavailable
--Sender is relaying mail -- probably using a commercial ISP while using a UM From:-address

554 sendall: too many hops 19 (17 max)
(the numbers may vary, and addresses may be included)
--There is a mail loop -- probably the recipient has mail forwarded to another account, which has mail forwarded back to the original account

>>> 553 Header Error
554 recipient's-address... Service unavailable
--This can be pretty obscure. There is a non-parseable line in the header, but finding it can be difficult. One example:

Resent-Message-ID: >"s3rGDD.A.saG.tTjM1"@earth.mat.net>

where a more standard message-id might look like this:

Message-Id: >>

>>> DATA
>>> 553 Header Error 2
554 recipient's-address... Service unavailable
--This "Error 2" may be a non-standard patch by the local postmaster to stop certain mail (e.g., used here to stop all messages with "Hi" in the subject line during a mass-mail battle)
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