On the other hand, if your only goal is to block anonymous SMTP, and
you agree that blocking all Tor servers is very overreaching, you
might instead try looking at the more targetted DNSEL service
available at
http://exitlist.torproject.org/
It lets you block _exactly_ those servers that relay traffic on given
ports to your address. For a more thorough rationale, and a fairly
detailed spec of how to make a compatible implementation, see
https://www.torproject.org/svn/trunk/doc/contrib/torel-design.txt