Apple Mail attachment problem

When I send an attachment with Apple Mail (using Lion), I've had people tell me that they keep seeing a line of text like this: [cid:0A24183C-5D58-4349-BCFC-0411526A8037]. They also get a winmail.dat file instead of the normal attachment. I've tried sending in plain text but they get the same result.

Any ideas?

 

This is a strange problem. If I send attachments to any other AOL user, they can open them just fine. It's just this one user who gets a winmail.dat instead of the attachment.