One of our servers, the one that hosts billing.rahul.net, resides at DigitalOcean, a cloud server provider.
A cable TV and cable Internet company variously known as Charter cable, Roadrunner, and Spectrum, is apparently rejecting incoming mail from all servers residing at DigitalOcean.
This company seems to be the only one on the entire Internet that has imposed such a wide block. They have no exceptions, period.
Everybody else blocks servers selectively, based on whether or not they send spam.
I could find no currently active customer who uses email hosted by this company. While we could redo our mail routing to still send mail to the company in question, it doesn’t seem worth the trouble, at least for now.
Our Classic Linux and DirectAdmin servers are not at DigitalOcean, so they are not affected. You can happily send mail to the customers of the company in question and it will reach. Only mail from support@billing.rahul.net is affected.