Restricting which email addresses a user can send messages to
To restrict which email addresses a user can send messages to, edit the user (in the Users page), and in the permissions tab for the user, leave the 'Allow sending of Internet Mail' box checked but and the email addresses you want to allow into the 'Target Whitelist' setting.
If you want to allow sending to all addresses except a few, then put the blocked addresses in the 'Target Blacklist' setting.
The procedure VPOP3 uses is:
* Is Outgoing mail allowed - if so, block the message
* Is ...