How we deal with the spam forwarding problem
We have a spam forwarding problem. Specifically, we forward spam, although not through choice; it's a political mandate not to impose filtering on people, and some of the people who don't filter forward their email elsewhere, and predictable things ensue.
We're dealing with this in three ways:
- we use a different source IP address when forwarding
spam-tagged email, splitting good email
traffic away from the spam so we avoid contaminating the former
with the latter.
This is especially important for places where a bunch of users may be forwarding their email, like Yahoo and Hotmail; this way we avoid having a user that forwards a lot of spam cause problems for other users who mostly forward non-spam.
- in order to eliminate as much backscatter as possible, we outright
discard bounces of spam-tagged email. This is definitely not RFC
compliant but is the lesser evil in a situation with no really good
(It also keeps our delivery queues small, since otherwise they would fill up with all of the bad addresses that spammers use.)
- just in case, bounces go out from yet another source IP address, used only for bounces, so if we get blocked for being a backscatter source it will affect as little mail as possible.
Our spam-forwarding source IP address has already shown up on a few email reputation systems, although I believe not in any of the public DNSbls. This doesn't bother me, because I can hardly blame anyone who blacklists it; it is pretty much a spam source.
(Since I don't believe in tempting fate, it has an innocuous host name.)