If you are using an Android or iOS device with an eg: External bluetooth keyboard, you wont be able to currently press Ctrl Z atm “as thats not supported yet”, only very basic elementary support “like the up and down arrows” and “backspace key” for deleting emails when going through the subjects in eg: Horizontal view as per @Michal_Burger post below in Nov.
So the Ctrl Z undo might possibly work later down the track. So for now “you need to press the Undo button with your finger” when it appears if you accidentally delete or archive a message.
So because I’m using gMail that means I have no way of knowing what email I may have missed.
As you have Gmail, then you go to the “All Mail” folder which is the archive folder for a Gmail account as advised previously, You can then open the archived email, and then “press the 3 dots at the top right of the message” and select Move to and select your Gmail Inbox folder again.