Forum OpenACS Q&A: Re: References to qmail
Except for one ancient Post.Office (some commercial product) instance (which was serving all the arsdigita.com email for far, far too long), which I think was also finally replaced with Qmail, all the servers that I was ever aware of at aD were using Qmail as their MTA, as far as I knew.
There are other alternatives to Sendmail now (Exim, Postfix, Courier, etc.), but I don't think those were at all viable back then, and I'm really not sure how well they all compare vs. Qmail now. Certainly it's still a matter of debate - do a search here and you should see various old posts arguing both for and against Qmail and other DJB tools.
Personally, I find that the djb tools exemplify unix: powerful, stable, and user-hostile. Or rather, administrator-hostile. They tend to fail silently and without explanation, which may (maybe?) be appropriate in some circumstances but makes them brutally unforgiving of mistakes or even just faulty assumptions while installing.