[Dovecot] IMAP only for some users or via webmail

Hello!

First, sorry for my English...

I'm a newbie with Dovecot and I have some questions,

I'm testing Dovecot 1.1.11 in a virtual machine. I wish in the future, migrate this to a real server.

Ok, but i need this:

1. A group of users can use IMAP or POP3.

2. The others, only can use POP3. But, this users can use IMAP only with a webmail (the webmail is in the same server that Dovecot)

It is possible? How?

Thanks!

Federico Ch. Tomasczik