While I have never professed to be a SharePoint or web anything, I was able to help one of my peers get mail delivery between Exchange and SharePoint working in a lab. The SharePoint person was looking at a published article and wanted to see it in action in our lab. I was tasked with the Exchange pieces of the setup so I followed the doc and we were successful in getting mail flow to work.
However, a day later when they were testing in earnest, I was informed that they would receive the messages and could see them appear in the "Drop" folder on the MOSS SMTP server but there was no attachment. After looking into it a little further and sending several tests of my own, I concurred that they were not coming across even though the message size was right on target. I used Outlook Express on the MOSS SMTP server to take a closer look and noticed the "winmail.dat" signature in the header. Well, I’ve seen this before so I knew what to do and created the appropriate remote domain on the Exchange 2007 server to deal with this. It worked as expected and they were now able to receive and process mail messages and their attachments.
Here’s what I saw in a sample message indicating the problem.
And the header looked like this:
Now, after seeing this I created a remote domain and turned off RTF as seen in this image.
After creating the remote domain and sending test messages through, the problem went away.