Yeah, I probably shouldn't have included qmail in that list. I've never used it to do 1m+ delivery runs. So far I've only seen Ecelerity and Ironport do that. I included qmail only because I'm pretty sure you couldn't kill it by sending it a ton of mail to send out.

Neither Ecelerity or Ironport need two machines to deal with slow receivers. You can setup priority levels on different queues if you want to make sure one large mailing can't stop later mails from going out, but the defaults seem to do a pretty good job of that for both, in my experience.

-sam