Our Homestead-hosted emails are being rejected with a "message rejected AUP#SPDNR" error

Our Homestead-hosted emails are being rejected with a "message rejected AUP#SPDNR" error.  When I google that error, the search results say the receiving email server blocked your message because its Acceptable Use Policy (AUP) flagged an issue with the sender (SNDR). This usually means your email, domain, or sending server IP address was misidentified as spam.

Please correct this problem, since it does not lie with us.  Sending a dozen emails a month is certainly NOT spam.