It was more than 48 hours, but they did get back to me.
I'm not able to talk directly to the engineer, and have to proxy communication through a basic support tech. The message was that the greeting is wrong, and breaks RFC 5321, and left it at that. "Do that" to fix it, and get an HELO. But the error still is not making sense, as other services that check for greeting compliance (SpamExperts, for example) are not giving rejections.
In fact, looking at the error again, it was first rejected due to RFC 822 (in-addr.arpa), which then triggered RFC 5321 (though I'm not sure why). Many PTR (reverse DNS) use a simple 4.3.2.1.in-addr.arpa as the reverse of 1.2.3.4 serve IP. However, this is not required. Refer to RFC 2317, which explains classles in-addr.arpa. That's what
Namecheap is using for server default PTR. For example, 4.0-9.3.2.1.in-addr.arpa. Note the 0-9.
I don't think Optonline/Cablevision's services are configured properly, and thus are giving us false rejections.
I've reported this, and await their findings.
When you look up the PTR with online tools, as well as the greeting (HELO/EHLO), everything passes. And there are no other problems with any other service.
Decyphering RFC isn't easy when you've not really done it for several years. (ie, The server just works, easy to forget how to solve problems when you have none! Thanks
Namecheap!)