First of all there are a few things to check.
A) Is your mail server name(IP ADDRESS) entered correctly?
B) Does your provider require you to use an SSL encryption and if so do you have the information entered correctly?
C) Do you have the correct ports listed in your email program. (Usually port 25 for outgoing/smtp server)
D) Can you ping the mail server in a DOS prompt?
E) Can you telnet into the mail server?
F) Do you have a firewall/router and have you tried eliminating it to ensure this is not the reason it is being blocked?
If you have tried all these options and all fail, contact your internet service provider. The problem is on their end.
Jamie
March 2011