jcurran@mail.com:
You block port 25 until a customer says that they're claim to have setup a responsible mail submission agent and demonstrate the necessary clue density.
Then in all fairness block also port 80. A comparable amount of junk is sent using port 80.
This can be readily determined by having customer support mail a short form with relevant questions such as "Is your mail server RFC2505 compliant?", "Please list the mechanism used to secure mail submission to your server?", and "Are you prepared to handle SPAM reports for all email originated or relayed?" No problem for someone who knows what they're doing but enough to deter the random end user.
Ditto | sed -e 's/25/80/' -e 's/SMTP/HTTP/' -e 's/MIME/HTML/' :-) Cheers, /Liman