We've opted for OpenDNS filtering. In the filter categories I'm blocking web mail. I've built in bypass credentials and associated them with our policy. However, when you attempt to access some web mail from Safari (on a Mac), in particular gmail and yahoo, the page won't load and instead you get an error, "Safari can't establish a secure connection to the server "mail.google.com""
My goal is for staff to be forced to bypass when using these sites. InternetExplorer allows a security exception, followed by the bypass screen.