Question : Emails bounce with message "MX invalid #439"

First off, we're using Exchange Server 2003 with the user in question using an XP workstation using Office 2007.

Today, one of my users started getting his emails bounced by a contact he's emailed many times before. The messages came back undeliverable and included the following:

There was a SMTP communication problem with the recipient's email server.  Please contact your system administrator.
            from ourdomain.com MX invalid #439>

I've had him send to a couple of outside email addresses that I own, and he's had no problems there, and as I've stated already, he's sent to this recipient many times before. Any ideas as to what might be causing this?

Thanks,
SA6

Answer : Emails bounce with message "MX invalid #439"

Looks like the person you were sending to were using "BarricadeMX" the list of error messages relating to this are listed online here:

http://www.snertsoft.com/smtp/smtpf/reply.html
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553 5.1.8 sender <%s> from %s MX invalid #439
The MX list gathered from DNS is pruned to remove hosts that resolve to localhost, RFC 3330 reserved IP addresses that cannot be reached from the Internet, or have no A/AAAA record. This message is reported if the MX list is empty after pruning.
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This also indicates the same revers DNS does not match.