Diagnose "SMTP" checks availabilty of a Mailserver and sends a testmail.
You can use this test, to check your configured Appliance and follow the SMTP Steps to view detailed information to possible Errors.

Further on it is useful to test the Appliance for incoming Mailflow, before switching on Port Forwarding on your Firewall.

Use following parameter for testing:
Target Host: <configured IP Address of your Appliance>
Sender: <configured sender Address of your Appliance (i.e. sf-engine@local)>
Recipient: <configured Adminstrator Address>
Subject: <any subject>
Message: <any Text>

    Detailed information about SMTP and the Status Codes can be found on Wikipedia and RFC #2821 Status Codes.