SMTP mail servers by establishing a connection on the remote TCP port (usually port 25) and do a handshake. By handshaking, can verify that the remote server's SMTP protocol is working well.
An SMTP Mail Server rule requires the following parameters:
Host - Hostname or IP address of the server to be monitored;
Port - TCP port number of the SMTP protocol. Default: 25;
Send command when connected - As soon as connection is established, send a command. By default, no command string will be sent;
Response must include string - when connected, optionally send a command. Then wait for the response. The default response for SMTP servers includes: '200';
Timeout - Number of milliseconds before the check will timeout. Usually, a connection to the server will be established within 1 second. However, some slow/busy server need more time. Recommended value is 7000 milliseconds.