[Dovecot] Restrictions on User Basis

Hi,

I am running dovecot on Fedora Linux in our organization and use IMAP as well as POP3 protocols for mail retrieval. I want to impose the following user based restrictions:

(a) I want to permit only selective users to access either POP3 or IMAP server or both.

(b) The server can be accessed from LAN as well as WAN. However I want to allow only selective users to be able to access POP3 or IMAP server from the WAN side (or on the basis of IP addresses).

What do I need to do to impose these restrictions ?

Thanks,