In the early days of Athena, users' electronic mail addresses were @athena.mit.edu. These were transitioned to @mit.edu addresses in 1994, and @athena.mit.edu addresses stopped working in 1999. If you wish to contact someone for whom you only have an @athena.mit.edu address, you can try changing the domain to @mit.edu, if you believe the person is still at MIT. (For example, firstname.lastname@example.org becomes email@example.com).
If you know the person's name, yqou may also wish to search the MIT directory at http://web.mit.edu/people.html or call the MIT operators at 617-253-1000.