Specifies the mail server's IP Address or domain name. A port name can be appended with a colon and portnumber (smtp.mydomain.com:587).

A mail server is required to send an SMTP message and you will have to know the mail server IP Address or domain name, plus possibly authentication information (username and password) to connect to the mail server. Typically this information is user supplied or stored in a configuration file of the application.

Examples:
127.0.0.1
mail.mydomain.com
smtp.gmail.com:587