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).
Type "open mail.domain.com:25".
220 www.domain.com ESMTP
If you see that message then you are not blocked.
If you can't connect, you are blocked. We run an alternate mail server for clients who experience this problem. Try changing your SMTP port from 25 to 2525 inside of your email client.