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.
What Os are you running?
thanks, but I still have the same problem I can not open the application
What OS are you running?
Look in Application folder for Mail App, try launching it there.
Do a Get info on your Home folder, unlock the lock in the Get Info panel. Make sure it says you have Read & Write privileges.
Using the tiny gear icon, use the drop down to "Apply to enclosed items...