When checking for new mail, either you or your mail program needs to enter your StarNet username and password. This verifies that you are a StarNet subscriber and have access to that particular account.
Most of the commonly used mail server programs do not do any form of verification when someone sends mail. The mail server setup StarNet uses also falls into this category.
This lack of authentication became a problem a few years back. One of the common tricks for "spammers" (people or organizations who send mass unsolicited commercial e-mail) is to send their mail through mail servers that don't check to see who's sending the mail.
When a spammer sends e-mail through a mail sever, he typically sends thousands (potentially tens or hundreds of thousands) of e-mails. This not only affects the people who receive the e-mails, it can cause severe problems for the mail server being used.
To prevent StarNet's mail server to be used to this end, it has been configured to not accept outgoing messages unless the sender is connected to StarNet (via dial-up or DSL).
This setup prevents others from being swamped with spam from StarNet's mail servers. It also prevents spammers from affecting StarNet subscribers from getting and sending their mail.