>    If the user does not exist, then it  could be that he or she does not
> really have an account anymore, or it  could be many other things - like
> the mailer is temporarily  broken and will be fixed within  a day or so,
> or the site lost all its ids somehow for a short time, etc ...
>
>    That being  the case,  just bear  it and  it will  clear up.   If you
> delete  or nomail  the user,  they will  then have  to re-subscribe.   I
> choose to be nice to my people and follow up as best I can, not deleting
> the userid until  I am reasonably sure it really  doesn't exist.  I send
> mail  to the  postmaster sometimes  to  find out.   Several times,  they
> didn't even know they had a problem and thanked me for pointing it out!
 
LISTSERV could help with this if it allowed setting NOMAIL for
some specified number of days.  Then the list owner could easily
set NOMAIL for a few days, then delete the user if the problem
occurs again when the NOMAIL setting is automatically removed.