If only a couple of "mail" subscribers at site have a problem
with your list's distributions, then suspect userid or authorization
problems for those users and either unsub or NOMAIL them.
 
If all list subscribers at a site have problems, suspect either
a networking, mail transfer agent, or authorization problem.
 
Depending on the list's activities, and your personal willing to
deal with "bounces," act accordingly.
 
On a list with 1700 users and 20 messages/day, I have little
tolerance.
 
/P