The Mail app update in 10.5.6 is crashing for a lot of people. In my case it was the GPGMail plugin. If you are running plugins with mail, you’ll probably need to update them.
If you are running gpg with the mail app, go here and get the latest beta version of GPGMail (d55).
Mail.app has no supported plug-in model. Anything calling itself a Mail plug-in is an unstable hack. If you use them, you should expect to have to disable or update them any time the OS changes. It sucks, but that’s life.