At least some of the folks doing sysadmin work at aD had a
long-standing (and likely correct) belief that Sendmail was insecure,
un-configurable junk, and that Qmail was by far the best replacement
available. (I think Philip even had that in his book.)
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.
