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.)
