Have you checked with your email provider to see if there have been email server issues?
An example would be the email address of,
Billy@ClearTechInfo.com. "ClearTechInfo.com" is hosted on a server. I use the server details to configure it to Outlook 2007 and it works great. Then a few months later I can't send out emails. The server could have issues and is not sending out emails.
The other thing that came to mind is, Have you been messing with any setting or settings lately? Maybe someone else who has access to your computer has messed with a setting or 2?