Currently you can change the threshold of which a user gets a mailtip warning when sending to a large group of users (the default is 25). It will then display a mailtipstating "This message will be sent to about x users".

It would be great for security purposes to be able to set a custom mailtip message to warn them about sending emails to large groups of people and whether it is necessary for this information to be sent.

I believe this can be already if you have on-premise Exchange.