1) EXHAUSTIVELY search all your e-mail folders – ALL OF THEM, TRASH, SPAM, INBOX, ARCHIVES, everything – for e-mails that contain the bad address. Remember that if the bad address was one of several in a message, even if it bounced the address is still part of that message. And it might be in a reply that was sent to you.
2) Delete EVERY LAST ONE of the messages with the bad address. The ENTIRE message. If it's something you really need to keep, copy-and-paste the contents into a NEW message and send it to yourself. DO NOT JUST FORWARD THE MESSAGE to yourself or elsewhere – the bad address will still get dragged along with it. Make sure you do not copy headers – body text ONLY.
3) Double- and triple-check what you did in steps 1 & 2 so you're not wasting time because you missed one.
For all the steps, go here: remove-addresses
UPDATE: This is insanely tedious, but if it works, great!