I'm getting this strange error on my zimbra server occasionally. When an email is replied to, a bounce back is received and the message is never sent. This generally is occured when the user does exist, but the user (in this posting, valid_user), does exist and can generally send/receive emails.

<message>

Subject: Undelivered Mail Returned to Sender
Date: Tue, 13 Nov 2007 17:03:41 +0000 (UTC)
From: MAILER-DAEMON@mail026.bisx.prod.on.blackberry (Mail Delivery System)
To: valid_user@valid_domain.com



This is the mail system at host mail026.bisx.prod.on.blackberry.

I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.

For further assistance, please send mail to <postmaster>

If you do so, please include this problem report. You can
delete your own text from the attached returned message.

The mail system

<slysmasher@vzw.blackberry.net>: host ###.###.###.### said: 550
5.1.1 <random_user@vzw.blackberry.net>... User unknown (in reply to RCPT TO
command)

</message>

However, the strange this is that random_user@vzw.blackberry.net was never involved in the posting, so how/why is the message trying to go through a verizon mobile server?????

Also, in the log/mailbox.log file, the only indication of a message around that time is this entry: INFO [ImapSSLServer-5465] [name=valid_user@valid_domain.com;ip=###.###.###.## #;] mailbox - Added message id=267 digest=md5_hash mailbox=31 rcpt=:API:

The last part is unusual, rcpt=:API:.....??

If anyone else has experience with this, or a similiar issue with user unknown 550, please let me know soon. Clients won't like receiving this unusual message for much longer.