Go to the spec, Luke! There's a way of preventing spammers from abusing open relays, involving a couple simple tweaks to sendmail.
In RFC 821, individual recipients of an email message are specified by a RCPT command, one for each recipient. Each RCPT must by acknowledged by an "250" response. If the mail server introduces an increasing delay (similar to the backoff mechanism in Ethernet) to it's response after the first 2 RCPTs, the server becomes useless for sending spam. Similarly, it could slow it's response if the rate of commands which start a new message (HELO MAIL SEND SOML SAML) from a given IP rises above a threshold. This would allow open relays to exist, without making them useful to spammers. Obviously, this would not help vs deliberately spam-friendly servers, but it would save open relays, which are a social good in the absence of spam. Peter Trei
