Adam,
I currently am filtering 36 domains with no problem. One of the domains is on the other side of of the country. The biggest factor when filtering emails will be how large your keyword list is and how many connections at one time you get. At night after around 11PM PST I see up to 50-60 connections from foreign countires, 99% of it is from SPAMMERS. I guess they don't think anyone will be monitoring that late.

Out of 205520 connections / 191847 email attempts since May 14th, 176082 were blocked and 8480 were forwarded to my mail servers. Please note that these figures are within 5% due to having email connections coming in while I was writing them down.

In that time frame I have 3 clients ask to be removed from the filtering and after one day two of them asked to be added back to the filtered list. Once they saw how much SPAM came to them in one day it was enough to convince them that they did not want SPAM and if the senders did not have a verifiable address, tough luck. Better a missed email then 100's of juck mail a day.

My 2 cents worth ... In a typical 24 hour period, we see an average of 55,000 connections spanning all the 500+ domains we host.  So far, the server isn't even working up a sweat.  Our inline antivirus is what hits the machine the hardest.

BTW ... we parsed the last 7 days of logs .... 88.9% of the inbounds were dropped as spam from the RDNS and dnsbl's alone.  We have had only a handfull of "good" mail killed and those we all from improper DNS at the senders end.

das