>they all seem to suck in some fundamental way. FWIW, in Postbox it seems to be speed.

That's because Postbox is based on Thunderbird, which is not a path to creating a fast, lightweight mail app.

I can only think of a handful of desktop mail engines for the Mac (Mail.app, Thunderbird + children, Entourage, Sparrow). If mail clients suck, it's because more people need to write one.