'All Drafts' folder has disappeared - how can I get it back?

In the side bar Favourites of Mail I used to have an 'All Drafts' folder in the same way that I have an 'All Inboxes' folder and 'All Sent' folder. I have three emails accounts - each of which is used fairly continuously. The 'All Drafts' folder has recently disappeared - possibly I dragged it off by accident. Any ideas how I can get it back please?

Many thanks.