Jared Mauch wrote:

What I do is format my smtp headers such that a very simple regex can find mail with no reverse dns and dump it in a spam folder. I find this catches a lot of the messages.

If your server is using a heuristic that has the potential of offending your customers, you can make the blocking optional. More specifically, respond to the "spam detected" condition by writing a header that customers can check for. Once that header's in place, a customer can configure his mail client to accept, delete, or spam-folder the message. If a client wants these kinds of messages to be deleted on the server, that may not be too hard to do either, once the header is written. -- David