As a long term Mac troubleshooter, I’ve run up against winmail.dat issues on a reoccurring basis. Typically when I’ve encountered them, a Windows user is emailing a Mac user, and attachments gets converted into a file named winmail.dat. 100% of the time this is because the Windows user is sending the email in Rich Text Format (RTF) format, and the fix is to figure out how to change the Windows user’s email application to instead send in either HTML or plain text format.
I was assigned a support task recently, where this behavior was occurring when a Mac user was emailing another Mac user. I’ve never seen this before, yet I was able to recreate attachments getting converted to winmail.dat files, but only when this one Mac user was emailing another specific Mac user (who worked at a different company). The emails were not in RTF format, and if I had this Mac user email me the same attachment, it came through fine.
It took me a while, but I eventually tracked the cause of this problem to a setting on the client’s Exchange server. On the server, I opened the Exchange Management Console, then went to Recipient Configurations->Mail Contact. Listed here was an entry for the Mac user that was having problems receiving attachments (the one at the different company). I modified this entry, and set “Use MAPI rich text format” to never. This fixed the problem. So apparently, the Exchange server was converting the email to RTF format because an entry for the recipient has been configured like this. Crazy.