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:
(a) Click Start Button (Windows Desktop)
(b) Click Run
(c) Enter \"telnet\"
(d) 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.