Mail quits unexpectedly

NVRAM RESET INSTRUCTIONS:

1. Shut down your Mac.
2. Turn on your Mac, then immediately press and hold these four keys together: Option, Command, P, R.
3. Release the keys after about 20 seconds, during which your Mac might appear to restart. For example, you might hear a startup sound more than once, or see the Apple logo more than once.
4. When your Mac finishes starting up, you might need to adjust any system settings that were reset.

SMC RESET INSTRUCTIONS:

1. Shut down your Mac, then unplug the power cord from the back of the Mac NOT the wall!
2. Wait 15 seconds, then plug the power cord back in.
3. Wait 5 seconds, then press the power button to turn on your Mac.

Rebuild the mailboxes

1. Open the Mail App
2. Select a mailbox in the side bar
3. In the Mailbox menu select Rebuild. Do this for each mailbox you have.

If all three processes fail then please advise and I will provide instructions for an EtreCheck report on your system.

Try some basic resets such as restarting in

Safe Mode

and also

SMC Reset

and also

How to reset NVRAM on your Mac

if those fail, then please rebuild the mailboxes.