mail/DNS question

[Sundar]

Hello,

I have a RedHat 7.2 server running as my mail and DNS Servers, among 
others like Apche.... Recently, one of my friends asked me to host his 
website and mail. So, I went ahead and created a virtual server in 
Apache(had to create Master Zone record  in DNS) and also created a 
user(for my friend) to allow him access mails on the system.

The problem is that since I have two domains defined on the system, the 
users are able to send and receive mails using both the domains. I want 
to restrict the old users to use only the old domain and my friend to 
send only using his domain name.

How do I do it?

Sundar