|It's been a while since I've seen this, but I think the MAC users should be able to do an "Open with" type command and choose the correct program for the .dat to open in.|
Not a permenant fix, but a work around perhaps.
I seem to recall something about the format of the recipients email address in your address book causing issues like this. Something about the email address being set as RTF maybe.
Sorry, I know that's all a bit vague, I'm sure someone will be able to give you a definitive answer though.