Our SMTP servers are configured as secure relays. This means that you cannot simply refer "mail.yourdomain.com" as an Outgoing mail server unless you successfully log in 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, it means that you haven't logged into the POP3 server at your domain, before you tried to send through the SMTP server at your domain. To log in, you need to check for mail first.
In addition to this, you might not be able to send regardless of if you check mail successfully. Some ISPs have it set so that their users cannot use an outbound SMTP server to relay messages. If you are still experiencing problems sending through your domain, and are certain that you have logged in via POP3 to an account at your domain, you might want to contact your local ISP to see if they do allow their users to use outbound SMTP servers.
Several large ISP's are in the process of blocking all access to port 25 (the SMTP port). They do this so that you will be forced to use their outgoing SMTP servers. Instead of using mail.yourdomain.com as your outgoing mail server, you should use the SMTP servers that your ISP provides you with. Or you can always use PORT 26 in your email software.