> But suppose I want to rescue one of those addresses
> from being automatically deleted?

A number of good approaches have already been discussed, and each of them is
suitable for some circumstances and not others. I am merely adding another
approach to the pile here.

On my lists, losing a subscriber unnecessarily is a significant concern. My
biggest list is still only around 900 subscribers, so I can expend some
personal effort on the few trouble cases.

I run with Auto-Delete= Yes,Manual most of the time, switching to
Auto-Delete= No when I need to collect some complete error messages to help
a subscriber show his IT gurus that they don't know what's happening in
their own domain.

When bounces for an address are clearly not a transient incident (usually a
couple days on the summary, with a bit of extra leeway for weekends), I
conduct a direct test by sending a note about the problem. If it bounces (or
I get no reply and the errors continue), I set the subscriber to NOMAIL and
start sending a canned notice that I have done so. I gradually increase the
interval between notices, always checking subscriber options first, because
the easiest way for a subscriber to tell me it's fixed is to set the
subscription back to MAIL.

Eventually I give up, but the subscription has by then been failing tests
for three weeks--usually enough to distinguish between a dead email address
and a problem that developed during someone's vacation--and delete the
subscription, but in the meantime the NOMAIL setting has reduced the amount
of extra fooling around on our servers.

I'm not saying that Bob Kosovsky's approach is wrong, it's just suitable for
different sorts of lists. And his is more than twice as big as mine, too.

Hal Keen