One of the recent Mail & Calendar update brought much-demanded linked inboxes feature to Windows 10 Mobile but seems due to some issues faced by users it was removed with a subsequent update. Now, Microsoft has clarified that they are in final stages of refinement of this feature and it should make a come back in future and that it was pulled temporarily to fix a bug.
@marcthepunk Linked inboxes is in final stages of refinement before we send it back to Insiders. Temporarily pulled it back to fix a bug
— Gabriel Aul (@GabeAul) November 20, 2015