Forwarding issue

It's not really a smartlist issue, but more of an administrative issue. I seem to get the occasional user that subscribes under one address, but then forwards the list emails from that address to another. Unfortunately the address that message gets forwarded to gets full or disabled and the servers that handle the bounce loose the original headers along the way. This makes it hard to for the smartlist to unsub an address that it does not have. Sometimes I can find some clue in the header that lets me figure out the correct address to do it manually, but I've seem to have gotten a bounce that has little info. Does anyone have suggestion for tracking down the `pop` box that is forwarding the email?