Squidguard, Transparent Proxy and Webmail

[mrpfsense]

All:

Is squidguard effective in blocking webmail services in Transparent Mode or non-transparent mode?

We have configured Squidguard but are seeing users accessing gmail and other webmail services. I've seen messages where some say squidguard is ineffective in blocking these https webmail services.

Would I see better results if we switch to non transparent mode?