How to protect mail from data leakage?

Question

Information Security

Good afternoon! Our company has a gmail account used within the company, and there were cases when employees sent confidential files to their mails, henceforth, in order to avoid such, the authorities want to make sure that messages are sent only to addresses from the address book.
There is a falcongaze, if you know how to organize it with it, please send answers

Answer 1

Protecting confidential data from being leaked is, I'll tell you, a serious task :) Usually, an entire information security service, or at least one person, is involved in this.
On all sorts of free mailers there is no protection against draining. From the word at all. Including from the drain to the mailer itself - Google in particular openly admits this.
To fully protect against a drain, a lot of work is needed, especially administrative, although there will be a lot of technical work. For example, a trade secret regime should work in an office - so that it can be presented to employees who have already pulled documents.
Your own mail server is required, SMP is also desirable