main in box stopped showing new maill
apple mail stopped showing new messages
iMac 21.5″, macOS 10.13
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.