Some parts of the ACS, and I guess OpenACS use qmail-inject for mail delivery. At the least, you would have to track that down and try to get ns_sendmail to do the job.
If the ACS webmail ever gets ported to OpenACS, you will probably need qmail to use it.
I don't mind plugging great software: install qmail, daemontools, ucspi-tcp and djbdns. Checkout
http://cr.yp.to/.
There is an up-to-date installation guide for qmail at:
http://www.flounder.net/qmail/qmail-howto.html.
