I've recently migrated from 2003 to 2010 and started to receive winmail.dat attachments on a lot more messages. It is not always from RTF messages, I have tested and we are receiving them on HTML messages also although not all the time. Attachments are also being changed to winmail.dat files on some messages too. This is on messages received here not sent out. Curious if anyone else has seen this or knows something else to check. Thanks,

On messages that you receive? It's more often caused by something on the sender side (as you seem to be aware). Can you contact any of the senders? What email client are they using? What email client are the recipients using? Are these messages coming from the same sender or are the senders from the same organization? If so, have they changed anything on their end? Just some thoughts...

I had a Mail Contact set up on my server. Every attachment sent to that contact was converted to a winmail.dat file. I opened the contact in the EMC and on the general tab I changed "Use MAPI rich text format" to "Never". if your old server is using a Contact to forward emails to you, I'd check that.