Try repairing permissions.
Remove the MAIL preferences from the username/Library/Preferences folder. Just drag it to the desktop and launch MAIL. You may have to reconfigure your accounts.
If that does not help, use the OS X installation disk to reinstall the MAIL app. You may need the help of an app called Pacifist to do this.
Do an archive and install of the OS.
In that order
My hard drive crashed. I started using my back up drive but my mail application will not launch. Any suggestions?
PM G5 OS10.4.11