Attachments are turned to DAT & NSF file

We have a very weird condition, One user sends *.Doc or *.PDF extention attachements and when a user outside of our organization get’s them the attachments are turned into a *.DAT extension and in some instances to NSF extension files making them unreadable. User has XP office Pro, we have Exchange 5.5 with trendmicro scan mail for exchange.
Any insight on this would be appreciated.