Preventing sendmail

Hi all, I'm trying to prevent PHP scripts from being able to send unauthenticated email through my server. I've read to change the following: Tweak Settings: Prevent "nobody" from sending mail [?] [ON] However, when installing this test PHP file (see below, replaced email addresses with XXX), the email is still being sent. Any ideas?

Thanks in advance. [COLOR="silver">- - - Updated - - - Also worth noting I have set up a limit of 25 emails per domain, but i can send as many messages as i want through this script.