which means either you or the isp are not receiving at this time. Now the ISP depending on how you are testing inbound might think it is a email relayer. For example when I was with the FAA in Alaska email outbound from our desks went thru a proxy to outside addresses that were being returned as undeliverable the ISP had a rule setup that proxied email was probably relay mail (spam) and would refuse to forward to the rest of the world.
thanx,
bill