Mail rule one to one

Hi I want u create a rule for a user1 who should send mail to user2 only. He should not be able to the send mail to anybody in the mailing address. I tried by creating like this

When :

Sender is User1/Test

Except when:

To or Cc is user2/Test

Don’t deliver the message - send NDR full.

In this case rule is working but if i add another user along with user2 mail is delivering. please give the solution if i add multiple user in to or cc mail should not deliver to any body except to user2.

Thanks…