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The Cisco Email Security Appliance (ESA) is a SMTP gateway appliance - it listens on port 25 by default.
The appliance can be configured to listen on another port for smtp traffic instead of 25. (GUI>Network>Listeners>Edit Listener>Port setting)
Should your appliance be listening for mail on a non-standard port, you can perform a telnet test specifying the optional port in place of 25. In our example below our mail host is listening on port 80.
In order to use a port other than 25 when using telnet to connect to a remote host, simply specify the port number following the hostname from the CLI:
esa.example.com> telnet mail2.example.com 80 Trying 172.19.0.94... Connected to mail2.example.com. Escape character is '^]'.
Alternately, the AsyncOS CLI will prompt you to enter a host and port if you simply enter telnet, then follow the prompts.