This is a follow-up to the thread mentioning that SG handles 1GB of spam per day.
Josh observed that the volume of messages is not a problem right now. But there are a couple of things worth considering.
1. avoid launching a sendmail process to send outgoing messages. instead we can use a perl module (Net::SMTP?) to talk directly to the MTA.
2. doing some filtering at the MTA. if we can discard some messages as they come in, it will be more efficient than the spameater processing which comes later. before we can implement this idea, it would help to have some statistics on the incoming spam: what fraction come to expired addresses, what fraction to non-existent users, ...