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.
This entirely depends on what email server software you are using more then likely. – Ramhound – 2014-03-14T10:45:40.850