Quit Mail
1. Open Mail while holding down the Shift key. This prevents it from opening the previous mailbox configuration. It can bypass a corrupt piece of mail.
2. If that doesn't work try disconnecting your online connection then open.
3. Trash the following file:
your home directory / Library / Preferences / com.apple.mail.plist
and empty the Trash
4. If none of these work, make a new user account on your Mac and try to run Mail from the new user. If the same problem occurs, your Mail program might be corrupted. Run Disk Utility to repair disk permissions or some other repair program like Disk Warrior.
If you have your original install disks you can use Pacifist to extract just Mail for reinstall.
http://www.charlessoft.com/