Sometimes there is a "daemon" feature on the receiving end that attempts to deliver for a few days. You may just have to live with it until that expires.

You could also be receiving spam with the same message of "undeliverable message" or whatever. Are you sure it's the same message you sent? The trouble is, if you open one that isn't the one you sent, you're likely to be confronting someone's nasty porn or penis extension ad. Good luck in figuring it out! They "spoof" legitmate mail bounce messages hoping you'll open it to see what you were sending that didn't make it to the recipient.

SRS