General mail bandwidth question...

Question

We are using a software spam solution on a separate server that filters the incoming mail by domain, then sends it on to OSX Server mail. We only have about 100 users.
We have what we consider to be a fairly good connection to the internet, 10MB x 2MB, yet it seems like the processing and handling of email is using a lot of our bandwidth.
We think it is because of the large amount of spam we are handling. We are averaging about 55,000 messages per day incoming, 75% of which is quarantined as spam, the rest being delivered to the users. That comes out to about 130+ emails per account per day that are potentially spam.
We will be ordering a new spam appliance shortly that will bring the catch percentage into the 99% range, but here is the question:
We obviously must accept the incoming mail to process it for spam, thereby utilizing bandwidth. Are there methods we can use to detect and stop the flood of spam, maybe at a TCP level before it takes up bandwidth?
The spam has just gotten ridiculous. It is up 8 times from a year ago. Looking for thoughts or directions to look into.

Answer 1

It only gets worse. Over 92% of the traffic to our machine (130 domains, 1000+ users) is spam. About 80% of that gets nailed at the RBLs, half of the rest by our filters, and the rest end up in user boxes to be postprocessed by SpamSieve (or whatever Qurb became for our Windows users). Apparently it is still against the law to exterminate spammers, and there seems to be a large percentage of the breeding population who still purchase their promotions, or can't secure their Windows botboxes, so through the magic of compassionate capitalism and certain lame OSen, spam is not going to die anytime soon.
You could offload your processing by using the likes of Postini or other services, or the spam appliance you mentioned. Beyond that, or my very tongue-in-cheek suggestion of vigilantism, there is no cure.