className =? "mutt" --> liftX (doShiftEmpty "email")
Oops, slightly wrong; liftX will propagate the result back, which in this case is () which is not compatible with (Endo WindowSet) as expected by ManageHook. The correct fix is
> appName =? "mutt" --> liftX (doShiftEmpty "email") >> idHook
which discards the () and substitutes an appropriate do-nothing Endo WindowSet.