Growl was causing Mail to crash if you installed Safari 4 beta and this is the problem with all these hacks, especially the input managers (not Growl though, I know)... they bugger up other apps.
Btw, it is because you have to disable all these hacks to be on the safe side every time Safari is updated that makes them very unappealing to use and it is why I would go with Firefox or Opera if I had to - Firefox is sensible enough to disable any incompatible extensions for you and Opera's features are built-in. No dicking about in the OS with hacks that can cause problems in more than just Safari.