> The usual cause of this is that A is subscribed. Mail to A is forwarded
> to B, which no longer exists. B's site bounces to listserv who not
> having B as a subscriber, can't autodelete A.
>
> Another possibility is that A's site bounces the mail with a different
> address. I.e. A = [log in to unmask] and the bounce is from [log in to unmask]
But passive probing still works as long as the bounces go to the proper
return-path address (owner-listname-etc).