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Because Stanford has a silent proxy picking up SMTP traffic, and running it through their security system.

http://serverfault.com/questions/382329/routing-smtp-and-pop...

Presumably zbjornson was sending messages from a network within Stanford, using vanilla SMTP. In that sort of topology, Stanford's network could easily intercept, monitor and modify SMTP traffic.

What the posters below said. If you use SMTP to send to an external server, and you are on Stanford's network, they intercept it. "Relay" might have been the wrong word.

He could have been sending them to a third party SMTP server from inside the Stanford network.