OpenACS doesn't require qmail (to be fair qmail is never capitalized). What id requires is maildir and verps. Postfix is capable of doing both. In fact almost every mail server can use those.
acs-mail-lite requires that mail be delivered into a maildir style mail folder. This can be done using the safecat package, maildrop, or procmail as the mail delivery agent for the MTA.
Postfix, Sendmail, and EXIM can also be configured to VERPs in the style of qmail.
qmail is used because it has these features available by default.
