"Tell A Friend" email security

[Tonearm]

I've set up a "tell a friend" type of emailing system on my site and I'm wondering what kinds of security features I should build into it. It basically takes your friend's name, your name, your friend's email address, and your message and sends an email to your friend with your message and a message from us. Any suggestions?

[HughMungus]

Require that the person sending the email to their friend receive and verify an email from you.