You need to ensure you're only accepting valid email addresses.
http://www.totallyphp.co.uk/code/validate_an_emai...ar_expressions.htm
It's also vulnerable to spam - people can continue to resubmit it etc.
I'd recommend implementing freecap, a PHP based Captcha library. A captcha is basically a server generated image containing a string that the user is required to enter before they can perform an action - protects against automated submissions etc.
freecap:
http://www.puremango.co.uk/cm_php_captcha_script_113.php
