On Tue, 29 Sep 2015, Florian Weimer wrote:
It's difficult to cause immediate mail bounces without running an MX host for the domain in question. Without immediate bounces, the issue might not be recognized in time (perhaps a bit of a stretch—but immediate bounces from your own smarthost make it easier to see that something is really wrong).
https://tools.ietf.org/html/rfc7505 A "Null MX" No Service Resource Record for Domains That Accept No Mail Paul
