Really appreciate the help -- and the education! I'm going to approach Suddenlink/Optimum with my new-found understanding that the modem/router they provided to me is PBL blacklisted and has no DNS Record. Perhaps that insight will trigger their fixing the problem they created for me.
(The backstory is that this particular modem/router is a recent replacement by Suddenlink/Optimum of a prior Suddenlink/Optimum-provided device that got fried in an electrical storm a couple of months ago. With the original modem/router I had no problem accessing Cox SMTP to send e-mail.)
Getting a new IP when swapping out a modem/router combo is pretty normal. Is has a different MAC address so DHCP has to give it a different IP. What isn't normal is must have pulled from a different IP block that doesn't have rDNS/PTR. That is up to Optimum. You might try posting on DSLReports to see if it is possible to fix so your not asking the impossible of their tier 1. Maybe try their direct forum so you can post personal info.
Thanks for everything! I have just completed a chat with Suddenlink/Optimum -- they promised a fix in 48-72 hours. Of course, I've heard that music before...
Keep us updated. If they can help that would be good to know for other users going forward.
