> I'm getting a ton of these, but I'm unable to do anything about them because > they have me competely baffled. I understand what the error is, but can > anyone help me figure out how to find out the identity of the sinner so I can > straighten him out. I haven't heard any complaints from any of my > subscribers. This list is only about two months old. > > The complete error message follows: > (snip) > > The mail you sent could not be delivered to: > 550 [log in to unmask] is not a known user It looks to me like it's what it says in the message: [log in to unmask] It's most likely someone who subscribed to your list and then closed their account. Is there some reason why you think it's not that address?