Most academic and public institutions keep "phone books"
of their students' and employees' E-Mail addresses.
If you know where the person you're looking for works or studies, contact his/her server and look through the phone book.

Another option is to connect to the Massachusetts Institute of Technology's Gopher server. MIT's server has a phone book that includes listings from many institutions.
Simply select the "Phone Books" option, choose the institution you're interested in, and type in the name of the person you're looking for.