[Dovecot] Dovecot's as ip/user based login filter?

Hello.

I'm facing a problem, I need to give access to internal mailserer for some people only, but can't figure out how to do it. Opening the 993 port for whole word gives access to everyone who have account.

Can Dovecot act as login filter or etc for that purpose. Or does anybody have idea how to do it?

-- Sysadmin