How does that work? Picture this: user1@somehost.com has subscribes to your list, but he has his email forwarded to user1@someotherhost.com. If his account at someotherhost.com, say, goes over quota, mail to that account will start bouncing. But where do the bounce messages go? Right back to you, the moderator.

How can I fix this? Unfortunately, you can't. Because you have no way of knowing what user on your list owns the bad address, you can't unsubscribe the right person. (And before you ask, no, we can't help either.) The best you could hope for would be to speak with the owner of someotherhost.com (usually postmaster@someotherhost.com) to see where email for that account is coming from, and unsubscribe him from there.