Mail quits immediately after opening on my iMac. anything I can do?

First, boot your Mac into Safe Mode. Instructions vary for different models:

Start up your Mac in safe mode - Apple Support

If you have the problem after rebooting normally, then try starting Mail with the shift key held. If that works, look over the emails and try selecting them one at a time to find out which is the rogue email. If one of them makes Mail crash, then try again, eliminate temporarily the preview window, and then select and delete that email.