I was confused by your answer regarding winmail.dat. The file cannot be opened traditionally, at least to obtain what the receiver believes to be valuable contents. Rather, it's used internally by ...

So instead of seeing the "rich text" formatting of the original message, you get a plain-text copy, plus a mysterious "winmail.dat" attached file. You can read these attachments with extra software.

I've got a few users that are now reporting that emails being sent out are showing up as winmail.dat attachments. This had been fine, and we switched over to Office 365 recently (in the last month ...