In VPOP3 a Mail Sender tells VPOP3 how to send mail out to another mail server, usually for sending mail across the Internet.
To create a mail sender, go to the VPOP3 Settings, then Mail Connectors. In the Mail Senders section on the left, choose Add New, and follow the wizard.
Note that in this version of VPOP3, each Mail Sender is associated with a Connection method. That is because often ISPs limit access to their mail relay servers to certain methods of connecting to the Internet - eg if you have 3 different dial-up methods, you will generally have to use different mail relay servers depending on which dial-up connection you use.