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:
- Click Start Button (Windows Desktop)
- Click Run
- Enter "telnet"
- 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. You should change your port to 26.