The winmail.dat issue has been happening to us between different Outlook 2003 users on the same Exchange 2003 cluster. So having Outlook on the receiving end is no guarantee that the attachments will appear correctly.
Having the sender change to HTML mode doesn't stop it either. Text mode helps but I'm not yet convinced that it completely eliminates the problem.
Microsoft recommended that we upgrade (replace) our '03 cluster with '07 running on Server 2008. So we just finished that, and I haven't yet heard any reports of the winmail.dat problem since then.
GroupWise rocks. I wish we could still run it.