Our SMTP servers are configured as secure relays. This means that you cannot simply reference "mail.yourdomain.com" as an Outgoing mail-server unless you successfully login via one of your POP3 accounts at "mail.yourdomain.com" before you try to send. This is a mandatory setting to prevent spammers from using our mail-servers as havens for unsolicited email. If you are getting a "relaying prohibited" or "disconnected by administrator" error, normally this means that you didn’t login to the POP3 server at your domain before you tried to send through the SMTP server at your domain.
In addition to this, you still may not be able to send mail. The reason why this may be the case is some ISPs have been set-up in such a way that their customers cannot use an outbound SMTP server to relay messages. If you are still experiencing problems sending mail through your domain, and are certain that you have logged in via POP3 to an account at your domain, you should contact your local ISP to see if they allow their customers to use outbound third party SMTP servers.