I wonder if:
- ezmlm (http://www.ezmlm.org/) fits your needs. An integration package of some sort would need to be written between the ACS and ezmlm, but uh, I shouldn't think that would be too difficult.
It's a dedicated mailing list manager written by Daniel Bernstein (who qrote qmail). It powers all sorts of "ez" mailing lists and knows how to both handle bounces and subscribe/unsubscribe users all by itself. (It would be very nice to integrate ezmlm's subscriptions with the ACS db alert system.)
- there is some way just to use qmail itself (or I think you're using postfix? (so some native postfix method?) to do what you want.
With qmail for instance, you could bypass ns_sendmail entirely, and just use native qmail commands to "inject" (qmail's terminology) a message into the queue. I know that qmail is supposed to be both very efficient and very smart about how it handles one message going to multiple recipients, so the end result should be that you can get your bulk mail out with neither an outlook CR/LF problem, nor requiring excessive resources from your system.
Without a bit of work, might not handle bounces correctly, although if I recall, very little handles bounces correctly due to ambiguities of headers and the way user's mail programs treat those headers.
Hmm. Now that I've written this, I have this feeling of deja-vu, I think we've participated in this same conversation about six months to a year ago. It's too bad we haven't made any progress on this....