I recently had question from customer who listed multiple mail serves in autosupport.mailhost but apparently only one of them was ever tried (and did not work, so no ASUP was sent). Are multiple mail servers supported? The version is 8.0.2 but would be interested for earlier versions as well.
Solving with DNS is a good idea. The ONTAP docs say that up to 5 mailhosts can be entered and it lists "they will be accessed in the order specified until one of them answers as a mailhost", One of the mailhosts might be responding, but not sending so another mailhost in the list isn't used.