Mail shuts down unexpectedly

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.

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.

can't open mail on my iMac os Catalina

Does it bounce in the Dock at all?

Have you tried what I posted above?