The throttle module can be used in several different ways, but the way that interests me in how it could be applied to mail is it slows down connection for people who are sucking up all the bandwidth. the more they hit refresh, the page loads slower each time.
If we get more than 50 e-mail from an ipaddress that is not ours, odds are they are a spammer. I would like to have sendmail have an exemption for addresses that are ours, and maybe a few mailhosts/ISPs that we think doa good job of killing spam. But for the rest, we would only allow one connection from a given IP at a time, and each e-mail we get from them, they would get a counter to make each consecutive connection from that IP address slower and slower. And finally, have a cap, where we stop accepting mails altogether if they ip address exceed the 24hr cap on messages.
Why? Over the last couple weeks the spam has gotten so bad that the load on our primary mail server gets too high and sendmail shuts down forcing the mail to fail over to the back-up mail host. the idea of throttling the ip addresses would ideally keep the load to a manageable level.