Mx2 is back in service on Ubuntu Artful 17.10.  I was unable to determine what they upgrade to 18.04 broke but was able to document the error and generated a ticket on Launchpad so Canonical is working on it.

     Mx2 isn’t needed from a capacity standpoint, it is just there to provide redundancy for Mx1 to receive mail in the event Mx1 is down.