Recently a customer was experiencing mail delivery failures:
Diagnostic-Code: smtp;554 5.6.1 Body type not supported by Remote Host
Turns out that the Windows SMTP service (what Exchange 2000/2003 uses to transfer mail) advertises 8-bit MIME, which isn't compatible with some mail servers (they were using Postfix). In any case it was causing a lot of bounced mail and unhappy employees.
After a bit of searching, the answer turned up in a Knowledge Base article, 262168.
The fix was mostly straight forward and I was able to use their commands verbatim. But there was one hiccup, the utility that is used, mdutil.exe, is not on the Windows 2003 Server disk and not available for download. I dug through my ever handy pile of disks and found a Windows 2000 Server CD which held the compressed file.
Once the fix was applied, mail detente was achieved between the OS factions.