MTA, as I see it, are a matter of taste for the most part. For most MTA fucntions each handles thems. Performance, configuration, etc. all seem to even out is some why.
Unlike your family, you generally get to choose your MTA :)
With that said, for bulk mailings and such qmail has the advantage of using VERP (http://www.jp.qmail.org/qmaildoc/RFC/RFCVERP.html)
easily.
I've found qmail's overall performace to be better (and email is a large part of what I do to pay the bills). Heck, what do you think most spammer's use? qmail.
But you could use any MTA and tune it to work with the bboard notifications. If your MTA is working for you, why switch.
(OpenACS) 3.x had the bulkmail modules (or at least talked about it) to handle unsubscribe requests. It might be helpful to had some bounce code handling to it so emails that are bouncing get their notifications blocked after x bounces (or x days). If you tune it further you could say and if one discussion is getting a large amount of bounces that you disable alerts for that dicussion also. That might help in reducing messages that are not be delivered and enlarging the queue.
Of course once you start to tune your mail sending/receiving/bounce procedures you'll start to lock yourself into a single MTA.
