Hi jdh9846,
Let's try to start up your iMac in safe mode, as safe mode does some system checks, and can sometimes resolve issues. How to use safe mode on your Mac
How to use safe mode
Determine whether you're using a Mac with Apple silicon, then follow the appropriate steps:
Apple silicon
1. Shut down your Mac.
2. Turn on your Mac and continue to press and hold the power button until you see the startup options window.
3. Select your startup disk, then press and hold the Shift key while clicking “Continue in Safe Mode.”
4. Log in to your Mac. You might be asked to log in again.
Intel processor
1. Turn on or restart your Mac, then immediately press and hold the Shift key as your Mac starts up.
2. Release the key when you see the login window, then log in to your Mac.
3. You might be asked to log in again. On either the first or second login window, you should see ”Safe Boot” in the upper-right corner of the window.
It may take longer to finish the start up, and you'll also see some differences in the display. This is expected with safe mode, and when you restart normally, they'll be gone. Test with the Mail app, then restart and test again.
Let us know what happens, so we'll know what troubleshooting steps to do next.
Thanks!