[Dovecot] Restrictions on User Basis

[Net Cerebrum]

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,