My email account on my iMac won't quit or open.

You can restart the computer by holding down the power button for a few seconds. Once you do, then restart in Safe Mode and then restart normally. If you use a wireless keyboard either use a wired USB keyboard to restart or plug in the charging cable that came with the keyboard to do the Safe Mode restart!

Restart then Start with this...

Safe Boot, (holding Shift key down at startup), does the problem occur in Safe Mode? Could take 10 minutes or more.

Safe mode attempts to repair Disks & clears lots of caches & loads safe Drivers, & prevents loading of 3rd party extensions, 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.