It is becoming more common that ISPs are blocking port 25 (SMTP port) forcing you to use their SMTP server. To see if you are blocked:

1. Click Start Button (Windows Desktop)
2. Click Run
3. Enter "telnet"
4. Enter "open mail.domain.com:25"

If you see a message like:

220 www.domain.com ESMTP

Then you are not blocked. If you can't connect, you are blocked and you will need contact your ISP and use there outgoing mail sever with there user name and password, however no one will be able to tell that you are sending mail over there mail server, the mail will look like it's coming from your domain.