when i want to shut down my iMac, message appear "mail interrupted shut down" but can't quit mail

Please restart the computer in Safe Mode per the directions in the  How to use safe mode on your Mac - Apple Support link. 

Note: If you are using a wireless keyboard please either plug it in using the supplied Lightning cable that shipped with it. If you are using a legacy keyboard or a third-party keyboard that does not have re-charging abilities, please use any USB keyboard. Also note that a Safe Mode restart may require you to hold down the shift key for up to 5 minutes before the computer starts in Safe Mode. You will know it is in Safe Mode when you see “Safe Mode” in the upper right of the display.

After restarting in Safe Mode, then restart normally and test.

This happens to me (M1 24” 12.5). If you can’t shut mail down from the dock try Force Quit.

I just get into the habit of shutting mail down before I shutdown the iMac