That's pretty bad, considering that you are now sending mail as
you@cox.com from your school's SMTP server. What's to keep me/your uncle/joe down the street from sending email as
you@cox.com? One solution to this is to block relaying on the SMTP server, which I guess your school didn't do, or to use such other tricks that mail servers sometimes use, like waiting for you to authenticate to read mail before allowing you to send mail, or using extended protocols that allow for authentication such as ESMTP. Scary to think that someone could be sending email as someone else, and the only way to really be sure a message is from who it says it's from is to check the headers (something that most people on the internet don't know how to do)...