Exchange Server 2007 Error 550: mailbox unavailable

From an end user account in our domain, our Exchange Server repeatedly attempts to send an email to a recipient with the following (modified) address:  (The domain name is correct.)  The end user continually receives a bounceback back message, as follows: #550  Requested action not taken:  mailbox unavailable ##

Earlier in the day an email FROM a sender in the "" domain was detected by our firewall as containing spam.  It stripped the spam but allowed it to continue to our Exchange Server, which allowed it to be delivered to the end user's mailbox.  At that point, the situation began as described in the first paragraph.

Thus far, the end user has counted 55 instances of the "Undeliverable" message from our Exchange Server.  The strange part of this is that the "" email address changes with every bounceback message.  The only constant is the domain remains the same, i.e.,

We've run Anti-Spam and Anti-Virus scans on the end user's pc.  It is clean.

Please advise
Who is Participating?
Will SzymkowskiSenior Solution ArchitectCommented:
I would be checking that you are not sending large amounts of spam to the internet. If you are you could be blacklisted.

Check your Outgoing mail and see if you have a spike in mail to the internet. If you are what I would do is block port 25 on the firewall and disable your send connectors in Exchange. From there you can then see who or what computer this is coming from, within the firewall logs.

I would also be checking to see if you have already been blacklisted from spam bots on the net.

If you know this is coming from a specific mailbox then what I would do is Export the mailbox to PST,. Disable the mailbox in Exchange and then create a new mailbox for the user and see if the issue persists.

If the new mailbox is working fine for the user, you can import the PST back into the new mailbox.

baleman2Author Commented:
Other than the response from Will Szymkowski, there have been no other contributors to the question.  Mr. Szymkowski's response, though valid, was not helpful.  Our Exchange Server is not blacklisted.  The other solution he suggested is not viable.

Since the email came from Germany, I used "geographical" filtering on our hardware firewall to block all emails from that region and resolved the problem myself.

That is why no point were rewarded.

This question can be closed.
Will SzymkowskiSenior Solution ArchitectCommented:
That is fine. Basically if you feel that i assisted in any way you can award points accordingly or grade how you like. If you feel that i have added absolutely no value then you can just close this question.

baleman2Author Commented:
Can't seem to close the question without awarding points.
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