Hello Michael2012, I hope you are well. The error message in the bounce mail would usually be '421 Too many concurrent SMTP connections; please try again later.' The error message denotes that the connections to the recipient mail server have exceeded the limit. This high traffic can sometimes be a temporary problem due to a spike in the connections.
I think depending on the type of hosting account you are using that will dictate the relay server address you will want to configure your application to use, here are the list of examples.