Anonymous Mail via Port 25

Awhile ago, someone mentioned that at some colleges & universities, the dialup lines don't require you to log in immediately. Rather, they let you specify which system to telnet to, and then you log in there. Telnets are usually limited to machines in the University's domain, but they often don't restrict what ports you can telnet to. So you can telnet to port 25 and send an anonymous mail which would be very difficult to trace. Does anyone know of any such systems? Anywhere in the U.S. or Canada? I'm thinking of using such mail ports for a new anonymous remailer I'm working on, and I want to make tracing the mail as difficult as possible. Please post modem numbers...