I have a single user on my SBS 2008 server that is getting winmail.dat files from random people instead of attachments or calendar appointments. If the same people send the attachments to other users on the same server they come through correctly. I also created a new account for this user and have it forwarded to his main account. Any attachments and appointments sent to this address show up normally in his main account.
After doing this I tried backing up his files and email, deleting the account and recreating it then restoring all of the data. Same problem!!! The winmail.dat files continue to show up :/