Mark as Done is how you say there is nothing left to do with an email. It is filed in the Done column of your Later Board, so you have a running record of what you have dealt with.
Nothing is deleted. The email is kept in full, and BlueMail marks it as read at the same time, since you are finished with it either way.
The email itself stays where it is. Done is a status rather than a move, so the message remains in your inbox, marked read and tagged as handled, and nothing is deleted or filed away out of reach.
Changed your mind, or marked something too early? Set it Undone and the status is simply cleared. See An Email is Marked as Done by mistake.
For the ways to mark one, see How to Mark an Email as Done?
