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 the recipient in the Target Blacklist - if so, block the message
- Is the recipient in the Target Whitelist - if so, allow the message
- Is the Target Whitelist Empty - if so, allow the message
- Block the message
So, if the Target Whitelist is empty, VPOP3 will allow sending to everyone, except those addresses in the Target Blacklist. If the Target Whitelist is not empty, then VPOP3 will only allow sending to people in the Target Whitelist.
You can use * and ? wildcards when creating entries in the Target Whitelist & Blacklist. * matches any number of characters, ? matches a single character. So, to match a whole domain, use *@domain.com