When I perceive that when proprietary-based subscribers are
getting full mailboxes such as those that occur on AOL.COM
or COMUSERVE.COM, I just delete them.
 
This sends the "correct" message to the subscriber, if they
should ever happen to return to processing their e-mail,
that they have a responsibility to act before they go on
vacation.  (Of course they could have been in an accident.)
 
Non-ISP subscribers e.g., .EDU or .GOV, I might just SET NOMAIL.
 
If the user has an automated vacation mail-bot, I usually
just SET NOMAIL.
 
/Pete Weiss at Penn State