The only email provider that I know of that uses an "All Mail" folder is Gmail. It's recommended for most people to hide that folder because it's not useful outside of Google's web mail interface. It shows by default when you set up Gmail, but if you have previously hidden it or want it showing, right-click on the account name in the left pane and choose "Subscribe". The context menu will show all available folders for that account - make sure "All Mail" is checked.

In case you're referring to Unified Folders, that option is found in the menu item <view | folders unified folders]. < p></view>