Using VRFY/EXPN
$ telnet/port=25 cs.arizona.edu
Trying... Connected to OPTIMA.CS.ARIZONA.EDU, a SUN-4/75 running SUN4.1.1.
220 optima.cs.arizona.edu Sendmail 5.65c/15 ready at Fri, 4 Dec 1994 14:21:53 MST
550 live-music... User unknown
250 <face@dirnet.ccit.arizona.edu>
221 optima.cs.arizona.edu closing connection