Can I recover an email after I have cleared it from junk folder?

Not easily. You would need to have a Time Machine backup that hopefully contains that particular email (Time Machine only backs up every hour; if the email was received and deleted between 2 backups, it won't be available) and you would need to restore the entirety of your emails as you can't restore just one email.

Can you ask the person that sent this email to send it again to you? The email would be in their Sent folder.

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