You need to use the mconnect program to verify mail aliases. In the following example, the mconnect program opens a connection to a mail server on a local host and enables you to test that connection. The program runs interactively, so you can issue various diagnostic commands. The entry expn sandy provides the expanded address, sandy@phoenix.example.com. This entry verifies that messages using the alias sandy will also be delivered. For more information, see the mconnect(1) man page.
% mconnect connecting to host localhost (127.0.0.1), port 25 connection open 220 your.domain.example.com ESMTP Sendmail 8.13.6+Sun/8.13.6; Tue, 12 Sep 2004 13:34:13 -0800 (PST) expn sandy 250 2.1.5 <sandy@phoenix.example.com> quit 221 2.0.0 your.domain.example.com closing connection %