Have you run software update to bring you up to the latest version of Mail 4.6, amonst other updates.
Run this update, even if you have it will not harm anything to run it again.
I am assuming your are running 10.6.8, if not run this update and it will bring your Mail up to date.
Go to Mail preferences and check that all settings are set correctly.
Also check thru this, sometimes it will help you find the issue>
Also try going to Mail preferences/Accounts, advanced tab and make sure that this account is enabled is checked.
If still no joy, contact your provider, they may have changed something in set up that they did not tell you about.
Hope this helps