Flagged Mail Continues to Return Despite Trashing

I recently upgraded from an old MacBook to a MacBook Pro. When I completed setting up my mail box there was an old flagged message that was sent from my email account back in June when my email was hacked. I have deleted the message in my old email inbox and in my web email box, but for some reason it continues to pop up in my new mail box. I've deleted and emptied the trash several times, but the flagged message still returns. Is there anything I can do to permanently delete this message?

Any suggestions or tips are greatly appreciated.