Can't open or close Mail

Start with this if IntelMac...

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

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.

Please tell us specifically what happens when you try. Has anything been added or changed prior to this occurring?