Mail app opens new untitled windows spontaneously - they won't close or go away

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It looks like you have multiple e-mails selected and opening at the same time.

But it's hard to tell with the menubar drop down in the way.

To start with Quit and Reopen Mail.

Then grab the mail panel by the bottom right corner and drag it out to a much larger panel.

Next grab the divider line and drag in down to show more e-mails.

Then try select (highlighting) only one e-mail at a time.