Can't open Mail after reboot

Safe Boot from the HD, (holding Shift key down at startup), does the problem occur in Safe Mode?

Safe mode attempts to repair Disks & clears lots of caches, so if Safe Mode works try again in regular boot.

If still no good Disconnect from the Internet, then start Mail, look for the last rec'd eMail.

Does Mail crash if no internet?

Quit Mail & login to your WebMail in a Browser, if possible delete any later eMails there after the last rec'd one on the Mac.

If you have no POP accounts & they’re all IMAP accounts then in Mail’s Mailbox menu choose Rebuild.