Go to User/Library/Containers/com.apple.mail. Move the folder com.apple.mail to your desktop. You must move the entire folder, not just the contents.
Restart, re-launch Mail and test. If the problem is solved, recreate any required Mail settings and import any emails you want to save from the folder on the desktop. You can then put the folder in the Trash. If the problem remains, return the folder to where you got it replacing the one that is there.
If that doesn’t solve it repeat the above using Containers/com.apple.MailServiceAgent.
Information learned from Linc Davis.