I have a reseller on one of my servers who has a client who refuses to change his mail login credentials to name@hisdomain.com and is using name@host.servername.net to log on to his mail account using POP-SSL, IMAP-SSL and Webmail. The mailbox name in the primary domain account mailbox if it makes a difference.

I would not care so much but his bandwidth usage is not recorded right and he moves a tremendous amount of email (size not qty - not a spammer otherwise I would just terminate the account) I have asked nicely several times but it has never changed.

Is there a way I can block him from logging on to the server in this fashion. I can not identify his bandwidth usage on his account and this is my main concern.