I would say step one is: You take a very close look at the logfile for the SMTP service that delivers your list server mail, to see exactly what is going on during the interaction between it and the complainer's server.

>>> [log in to unmask] 08/28/01 11:41AM >>>
What do you do when someone complains that they are receiving the same message 250 times and it is not happening to you or anyone else?