How to organize secure mail in the enterprise?

Question

Email

It is necessary that users in the enterprise can only send letters to each other and that there are only a few users who can send mail to all other mailboxes. Also, in the case of privileged users, pre-moderation of their letters is needed. Ideally, it would be gmail for work, but I can’t find it or you can do it there. What are the options?

Answer 1

What are the options?

Install your own MTA in the local network .
Install mail clients for users with created local MTA accounts (+ who needs external ones) and restrict the creation of new ones.
To send to the outside through the local.

Answer 2

gmail for work...secure mail...

Am I the only one who sees the paradox in this combination? Safe mail will be on its own server - and then, if configured correctly. Gmail for work can't be secure. A-priory.
Set up your mailer and sort out the sending yourself. By the way, what kind of moderation do you want? After the fact, a copy of the letter to Comrade Major? Or should Comrade Major first approve?