> 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?