Blocking access to mail

Question

Computer networks

Colleagues, good afternoon.
Tell me if there is a way to immediately block all users of the office network from accessing the web mail of free mail resources, but without affecting the performance of other sites (for example, the proposal to block POST, PUT, CONNECT does not work), otherwise it makes no sense to ban them all. Firstly, there is no complete list, and secondly, they appear constantly, and besides, what prevents you from buying a domain and using it as a mail service.

Answer 1

No. I think. Web is the same but also web. There is no obvious indication that the site the user is browsing is an email service. Either ban specific addresses, or filter by the presence of some lines in the body of the pages ...
Even if there are some distinctive headers or something else, it is not a fact that they are present in absolutely all webmails.

Answer 2

What is the end goal? Minimize the risks of information leaks? Then such a “patch” method is clearly not suitable, because it is useless. It is necessary to plan the implementation of a DLP system in a company.