Mail.app problems after updating to 10.5.6
Symptoms: After updating to 10.5.6, Mail may encounter such problems as unexpectedly quitting, not being able to quit out of Mail and problems loading messages.
Solution: Apple has acknowledged this issue and provided an update “fix” for Mail. Run Software Update, if Mail Update 1.0 appears, go ahead and run the update. Reboot and test. [...]