Thank you for the link. I checked my duckdns.org subdomain and found that I am on a blocklist and my reputation is poor. I don’t even run SMTP. Comcast Xfinity, if that matters.

Fwd/Rev DNS Match Yes Last Day Vol. 0.0 Last Month Vol. 0.0 Block Lists 1 BLOCKED IN: pbl.spamhaus.org Email Rep. Poor