You are on a an ISP that blocks their users from using any SMTP server other than their own. Your solution is as follows:
1. To setup your ISP's SMTP servers for your outgoing email, you must click on your account settings in the email client you are using.
2. In the SMTP (outgoing mail server) field, you will enter the SMTP server address for your ISP, such as smtp.yourisp.com. You must then enter the username and password for your ISPs mail server. For a list of ISPs and their SMTP server address.
3. In Outlook Express, this is at the bottom of that same tab. You must uncheck My server requires authentication and click on Settings to enter the username and password for your ISP.
Your email will still be from your own domain name, the only difference is in the way it is routed throughout the internet. Instead of being sent through our servers, it is processed through your ISP. This allows your ISP to monitor your email in case of a spam complaint.