[Dovecot] int/ext mailserver

[Mr.Pine]

I 'm using potfix-dovcote as an internal mail server(local.myco.com)
and wan to run another potfix-dovcote as an external mail server
(myco.com) on Internet.75% of my users should only work internally(no
external mail) but the rest of users (25%)should be able to send mail
to external network! These users have the same username on myco.com. I
use smtp_generic_maps to map int username of these users to ext. one.
For eg. All email send by test at local.myco.com map to test at myco.com. So
anyone got these mails can reply them.
I know that getmail is the tools that can get mails from users
external mailbox and put them into int. mailbox.

1. I have a root access to ext mail server. But do not know my ext
user password!. How can I use getmail to move ext mail to internal
one?!
2. What is your idea about syncing users password in internal/external
mail server?! I think its needed for getmail!
3. How can I restrict my internal users to send mail only internally!?

I appreciated any help!!

TNX