> 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).