edwardspl@ita.org.mo wrote:
  

Hello Charles,

1, All Services in the same Server machine.
2, Prevent some users from sending ( though SMTP Server : Sendmail ) /
receiving ( Dovecot : pop3 / imap ) his or her emails.

Edward.
    

Really, I don't see how anyone can possibly help you with the level of
information you've provided, and to be honest the scope of this list
probably doesn't cover basic user management, so be thankful for any
advice you do receive, and at least provide enough details for people to
try.