I want to restrict my users to use a specifc contact list

[Toxie Case]

I want to create a specific contact group for a set of users so that the only people they can send/receive email is from that contact group.  Can this be done?

[JohnDoh]

Roundcube is only a mail client. I think it would be better to setup restrictions like that on the SMTP server so that they apply no matter what client is used.