The new (Azure IaaS) IP address that the server is on appears in a number of SPAM blacklists.  The entries into the SPAM blacklists pre-date my acquisition of the IP address.  I'm in the process of trying to get the IP de-listed, but it will take some time for some of them as I refuse to pay the extortion fee for "express" de-listing that some of them demand.

It's clearly frustrating for all of us, but we're getting there.

Thanks for your patience.