Apple Mail keeps opening empty windows on MacBook Air

Apple Mail keeps opening a multitude of empty windows on my screen and I cannot get rid of the problem.

I've so far

• restarted the macbook air (sequoia 15.5)
• deleted the "com.apple.mail.plist" file in the library/preferences
• force-quit mail and re-opened with shift pressed
• emptied the outbox and deleted mail trash bin

None of these suggestions worked. Has anyone got a better idea?

(PS: I read about "safe boot" but don't know what that is. Pushing down the start button only leaves me with a black screen)

[Re-Titled by Moderator]

Original Title: Apple Mail keeps opening empty windows