Outsourcing Email For An Entire Domain?

CritterNYC writes: "I was wondering if any fellow Slashdotters have experience outsourcing email for an entire domain(s). My web host includes mail hosting, but I'd like to be able to use ORBs or MAPS to filter spam (among other things)... and my host's fees for additional domains on the same account are a bit unreasonable. I'd probably only have a couple accounts with mulitple domains pointing to them, but may grow in the future. Has anyone found a good reasonably-priced company that does email hosting?" Mr. Critter is probably not alone -- charging for services like mail is one way that hosts try to increase their profit margins. Any ideas out there?