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Because it is easy I would try restarting the computer before doing anything else. That can often clear out little glitches like that.

Also, please confirm your macOS version as 10.11 as shown in your equipment line, or update that if it is different. IF the restart dos not help, knowing the exact OS verison will make sure we directly you appropriately.

Quit mail.

Safe Boot from the HD, (holding Shift key down at startup), does the problem occur in Safe Mode?

Finder>Go>Go to Folder, copy & Paste:

~/Library/Containers/com.apple.mail

Highlight all the contents.

Then right-click and choose "New Folder with Selection”. Do not move the new folder from this current location.

You can close the Finder window.

The next step will be to restart the Mac and test

After the computer is restarted, open the Mail Preferences to see if it opens properly.