I want to do imap access restrictions from the internet.

Managerial level user will only be allowed
imap access via the internet.

How to restrict users are allowed to log in imap
only a certain user from internet?

Current Condition :
- 1 mailbox server using zimbra 8.0.2
- 1 mta & proxy using zimbra 8.0.2

I have searched on google, but have not found the right solution.