Whenever an incoming fax came in and Panther's fax software mailed it to my specified email address, the attached PDF file would show up as plain text in its encoded form. The message was not properly formated as a MIME message because of the method used to send it.
The solution turned out to be ridiculously easy. The file /usr -> libexec -> fax -> faxnotify had this line which would mail out the entire MIME message as the body of another message, causing an email with no subject and the wrong sender:
stdout, stdin = os.popen2('/usr/bin/mail ' + quote(receipents))
Changing it to the line shown below causes the mail and attachment to go out properly:
stdout, stdin = os.popen2('/usr/sbin/sendmail ' + quote(receipents))
[robg adds: I have not tested this one myself.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20031029145203142