On your Mac, click the Apple menu in the very far top left of the screen.
Choose "Force Quit..." from the drop-down menu that appears.
In the new window that opens, select Mail in the list and click the "Force Quit" button.
Mail should stop running shortly afterward and allow you restart the Mac.
this in fact should not happen every time. Are you able to use Mail normally but simply can't Quit it as expected?
Can you tell me the exact version of OS X you are currently running, please?
Click the Apple logo in the menu bar at the top left of the screen
Select "About This Mac"
In the window that opens, look for the "Version" information. Please let me know what it says.
Shutdown your Mac.
Restart and, as soon as you hear the startup chime, press and hold the “OPTION” (or “Alt”) key at boot until the boot menu appears.
Then, select the “Recovery" volume.
(Alternative: If the above doesn’t work… Restart, hold COMMAND + R keys until you see the Apple logo and progress indicator. If neither of those work, try COMMAND + R + OPT.)
Wait for “OS X Utilities” menu to appear (DO NOT CHOOSE TO INSTALL)
Choose Disk Utility from the available options.
In Disk Utility, select your normal, bootable hard drive volume from the list in the sidebar (usually "Macintosh HD").
Then, Click "Repair Disk"
If it finds errors, repeat the process until it reports the drive is OK.
Next, click "Repair Disk Permissions"
Once the process has completed, choose “Disk Utility” > “Quit Disk Utility” in the menu bar at the top of the display.
Then, restart your Mac and it should boot back into your normal Desktop.
Now see if performance has improved.
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