Can outgoing emails from a webmail provider be blocked using network management software?

Question

This is probably a ridiculous question, but is it possible to only restrict outgoing email sent from free webmail providers (Gmail, Yahoo, etc.) in a corporate network using network management software?

This is assuming users do not have access to a VPN service to bypass network restrictions and that email is accessed solely using a web browser.

Answer 1

For web-interfaces users only need HTTP proxy to sent e-mails, so it would be impossible to block specific sites without traffic inspection.

Also, inspection of HTTPS traffic require performing some kind of MiTM - installing on user's machines your root CA and signing with it wildcard cert for you [transparent] proxy server, which will decrypt and analyze all passing HTTPS data.

So simplest way to prevent inexperienced users from using external e-mail services may be DNS blocking - install DNS caching server on your network, limit external requests on UDP and TCP ports 53 so only this server can do so and add DNS records for undesired domain's that point to some other IP, i.e. 127.0.0.1

This kind of blocking can be bypassed quite easy. It's far better to provide handy internal mail service and persuade everybody to use only it for work letters.