Deleted all emails in Mail app but still shows 11Gb of space used by Mail app, how to fix that?

I usually use Safari Mail rather than the Mail App, but either way, whatever you delete from the Mail App, should also remove emails from Safari mail. Do you use any other mail apps like Google Mail, or Firefox etc? You may have sent all your mail to the trash bin, but did you empty the trash?

This may identify your problem: https://support.apple.com/en-gb/guide/mail/mlhlp1001/mac

Try this app to search your whole drive for...

.emlx

(dot ee em el ex)

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http://apps.tempel.org/FindAnyFile/index.php

Hold Option or alt key when selecting Find to Find All.