After spending a fair amount of time yesterday getting my Mac Mini rebuilt as my desktop machine (it has spent the last 18 months of its life faithfully as the Gray Matter mail, web and everything-else server) I was most disappointed this morning when I couldn't get Apple Mail to stay up long enough to read a single message. After about an hour of googling I decided that none of the fixes seemed to be applicable to my situation.
So I decided that rather than try and let someone else solve the problem, I'd try and think it through. Being a new installation, less that 24 hours old, it's not too hard to go back in your mind through the step you made in building the system. So one at a time I started mentally trying to pinpoint the moment when everything went pop. Then it dawned on me. Before I put the mini to sleep last night I installed Growl. Even though their site simply advises you need OSX 10.4 or better (I have 10.5.1) there are evidently some problems with Growl 1.1.2 and OSX 10.5.1; specifically Apple Mail. In short, the mail plugin for Growl will cause Apple Mail 3.1, as shipped with OSX 10.5.1 to completely fail with a segmentation fault. After disabling the Growl mail plugin, all of a sudden all my e-mail came back to life.
Like most new software platforms, OSX 10.5, Leopard, is a steady evolution of its predecessors. However, this introduces a number of compatibility concerns for third party developers and OEM's. Given how Apple took an open source project called "Cover Flow" and integrated into their base operating system (and now made it even more pervasive in Leopard) it seems odd to me they haven't done a similar thing with Growl. It seems every Mac user I know has Growl installed if for nothing else than for mail and IM notifications. So get with it Apple, either built your own notification framework and user-space applications, or take Growl and do your Apple magic with it :)