You can view previous versions of any draft email and revert back to a previous version at any time. The version history goes back all the way to the creation of each email message.
A new version is generated each time a portion of the message (individual content block) or the entire message is saved and it's been over 5 minutes since the last save. If a message is saved within 5 minutes of the previous version then that version is overwritten with the most recent message.
A new version is also generated when an active login session expires while the email editor is open.
A new version is not generated when the order of content blocks change (drag and drop). For this you should use the Undo option.
You can access the email version history page using the "Version history" link from the message editor.