[tex@smtp-client ~]$ telnet smtp-server.example.com 25
Trying xxx.xx.xxx.x...
Connected to smtp-server.example.com
Escape character is '^]'.

MAIL FROM: <user@smtp-client.example.com>
250 Ok
RCPT TO: <invalid@bioinformatics.rit.edu>
450 <invalid@bioinformatics.rit.edu>: Recipient address rejected: User unknown in local recipient table