We have been experiencing problems for about the past 3 weeks sending pdf attachments in emails to some contacts, where the attachment will arrive as a winmail.dat file (prior to three weeks age we had not seen the problem).

I'm well aware of the longstanding problem with RTF emails delivering attachments to some non-outlook files as winmail.dat files, but the weird thing is the messages are being sent in Plain Text format.  We've also tried sending the messages in HTML, and still the recipients receive the attachment as winmail.dat.

The problem is only occurring with some recipeints, and it seems that what those recipients have in common is that they are using Windows Live Mail.

Anyone have any ideas why this behaviour would be seen on Windows Live Email accounts for messages sent in both Plain Text and HTML format?