I can send email as ANYONE in my GAL! How do I stop this?

[Kool-IP]

I have created a new user account in Active Direcory which I will only be using as a mailbox. I then added the mailbox into my Outlook so that I can see my Inbox and the Inbox of the other account. When I send an email to the other account it shows up in the Inbox. When I open that email and reply I can click the From button and it will open up my Global Address List. I can make this email look like it's coming from anyone on my GAL. If the users find out about this...not good. How do I lock this down?