Right, and that fork is the only version of qmail people still run, and the bug they found was extremely funny given Bernstein's original qmail design (it was, if I remember right, a popen(3) vulnerability --- something that never would have showed up in Bernstein's code, but that's what happens when code gets abandoned, it gets picked up by people who don't really understand it). But it's hard to charge that vulnerability against the original qmail design.
"works fine" and "has some compatibility problems" is a little bit of an oxymoron... I understand what you're trying to say, but that does mean it's essentially unusable, despite "working fine".
(I don't think anyone should run qmail.)