Preventing To/cc from certain address lists

[mcnuttlaw]

We need our users to use a certain address book only when using the Bcc option. In other words, they cannot use it for To/Cc but only when using Bcc.

This address book is for our internal document management system allowing emails going into our client folders. Therefore, we do not want to send these email addresses outside the firm.

This address book is in Public Folders.

Any ideas?