Undo Send is a feature available to all ProtonMail users of ProtonMail V4, currently available in beta. Undo Send provides a variable length window during which you can recall your email.
The feature is enabled by default, with a 10-second timer. To disable it or change the timer length, go to Settings -> General -> Messages -> Undo Send and select your preferred timer length from the drop-down menu. Setting the timer to 0 seconds disables the feature.
If the feature is enabled, once you’ve finished writing your email and hit the Send button, the ProtonMail composer window closes and a banner shows up at the top of your screen with the message: Message sent Undo.
This banner is displayed for the set number of seconds (10 seconds by default). During this time you can retrieve your message by clicking on it. Your message will reopen in the composer window and you can go back to editing your email.