While it does not answer what has caused the interruption to your outbound mail it is not a standard use of port 80 (the http port intercepted by Webshield) to run a secure SMTP connection on it.
I notice that the instructions for setting up outgoing mail for smtpout.secureserver.net give you the option of using port 80 or port 3535.
You may find it worthwhile to try port 3535, if that is successful for you it will avoid any potential for conflict with the Webshield.