How to delete a self signed certificate and stop digitally signing mail

I was messing around with Adobe signatures and created a self signed certificate on my imac which I see now is sending out emails with a note "Unable to verify message signature". My email address is red on my iPhone and iPad.

I've tried deleting the cert in Keychain Access as well as deleting the mail folder in containers per this old post here but it doesn't work: https://discussions.apple.com/thread/7446759?answerId=29732228022#29732228022

Once I delete the certificate, it comes back even after rebuilding my mailbox.

I would like to get rid of the certificate and quite sending out signed email. Any help would be appreciated. Thank you.