Has the user attempted the Safe Start of Apple Mail ?
With Apple Mail Closed, Shift and click the Apple Mail icon. It is sort of like a Safe Boot but only affect Apple Mail and Not the whole computer
If still having issue with Apple Mail see below
1 - Suggest restarting in Safe Mode. This will perform a Disk Repair, clear cache files and only load Apple Software, extensions and fonts. The boot up will be slow and can take some time - Normal.
2 - Sometimes a Safe Boot followed by a Normal Boot will just put things right.
3 - Does the issue present in this mode ?
4 - If not - there could be something in the main User Account playing up. To further isolate this - Set up users, guests, and groups on Mac. Then log out of the Main User account and log into the dummy account and test again if the issue persists.
5 - If the issue is present in the dummy account - then, this appears to be a System Wide issue on the computer.
6 - If after performing each of the above steps is the order that have been presents and still have issues - please advise for possible further assistance.
