After years of testing, "Undo Send" became an opt-in Gmail feature in 2015; it's now turned on by default and can't be deactivated. Even if it's built-in, you'll still need to know what you're looking ...

For its 21st birthday, Gmail wants to make sending end-to-end encrypted (E2EE) emails much easier for companies in regulated industries. The goal is to “enable enterprise users to send E2EE messages ...