Winmail.dat

Exchange sends attachments embedded in a winmail.dat archive, which RT doesn't handle.

General this is an Exchange misconfiguration because winmail.dat is an proprietary attachment format used by Microsoft and should only be used internally. If Exchange sends an email outside to the world it always should convert the winmail.dat attachment back in the native format because you could not expect that on the other side is always an Exchange server that understand winmail.dat. ;-)

Solutions

In Exchange 2010, disable "Use MAPI rich text format" on the General tab of the RT contact.

In Exchange 2007, set "Use MAPI-RTF-Format" to "never" on the General tab of the RT contact.

From the RT server side, you can pipe mail through /usr/bin/tnef (after installing it) and then through rt-mailgate. This may not always work, and configuring it is left as an exercise for the reader.

More information

http://support.microsoft.com/kb/241538

http://support.microsoft.com/kb/138053/

http://issues.bestpractical.com/Ticket/Display.html?id=9073

http://www.gossamer-threads.com/lists/rt/users/93866

http://en.wikipedia.org/wiki/Transport_Neutral_Encapsulation_Format