External email acknowledge before Sending?

Hey All, need end users to acknowledge they are about to send an external email.

A popup box.

I have found this SafeSend , but does anyone have another recommendation?

Exchange 2016

An enterprise solution? We use Tessian where I am.
Hooks into Outlook, gives you a little “Hey, are you sure '[email protected]' should be on this e-mail? Everyone else is an internal contact”-type message.