I agree with David Sitman's recent posting favoring unsubbing
bounced addresses vs. the alternative(s).
 I immediately delete most bounced addresses these days, because some
systems generously shower me with rejections for up to 13 days
after the initial failure to deliver. Most often (always?) the
rejections include the full text of the rejected message. As
Yogi Berra said, "it's deja vu all over again."
 Perhaps patience level depends on one's job. If I were a comp.
center employee, I might feel differently. But to get rained on
often because someone hasn't bothered to sign off a list or
clean up their mailbox just isn't worth it. BTW the subscribers
haven't complained, and the Renewal feature seems to be
working well on biosph-l (set at 4-Yearly).
-dave phillips