It is becoming more common that ISPs are blocking port 25 (SMTP port) forcing you to use their SMTP server. To check 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 cannot connect, you are blocked. You should change your port to 26.