That's not necessarily a sure cure, depending on the hosting provider. RoadRunner (Spectrum / Charter) in the US and Shaw in Canada won't deliver emails from my domain hosted at Runbox.com (or sent directly from the runbox.com domain.) Spectrum's bounce message references an error code that translates to "Spectrum limits the number of concurrent connections from a sender, as well as the total number of connections allowed. Limits vary based on the reputation of the IP address. Reduce your number of connections and try again later."