I figured it out, in case anyone else runs into the same issue. I was trying to figure out why replies were being handled differently and I knew that the original header becomes wrapped in the reply header. So it occurred to me to try piping the message through the tnef decoder twice and viola…
rt-support: "|/usr/bin/tnef|/usr/bin/tnef|/etc/smrsh/rt-mailgate --queue ‘Support’ --action correspond --url http://rt.domain.com/
SteveFrom: Juan Mas [mailto:firstname.lastname@example.org]
Sent: Monday, June 07, 2010 12:16 PM
To: Steve O’Brien
Subject: Re: [rt-users] winmail.dat attachments only on ticket Replies
I just went through a similar issue. Though for me, if a user submitted a new ticket and were using Rich Text formatting, it would create the winmail.dat file. The only way I was able to fix it was to disable TNEF formatting in Exchange. If you look up TNEF and RT you will probably find some workarounds for your RT server, but I couldn’t get any of them to work, so resorted to disabling it completely on Exchange.