UPDATE »
As was suspected, 24 hours after this issue appeared, it turns out to be a Cox issue that has resolved itself, as I am now able to successfully connect to Cox's SMTP server to send email with my email client - with all mail account settings the same as prior to outage.
Very frustrating to say the least - NO THANKS TO COX - for the wasted day yesterday.
I will appeal to the Cox personnel that post to this forum to inquire about why this outage might have occurred. All techs or support personnel I spoke with assured me no changes were effected to their SMPT servers, when, at least in my case, most assuredly, something did change. With all the hardware and software involved, I know chances are slim the cause can ever be pinpointed, but I would appreciate some effort to assess so this does not happen again.
My advice to those who may run across this thread facing a similar issue, if you can, wait for 24 hrs to see if the issue resolves itself. Before going through the trouble of rebuilding, reinstalling, restructuring, and otherwise blaming a system that was working fine one day and then all of a sudden is not, wait it out for a day or two to see what happens, if time permits of course. Good luck.
Over and out for now - with hopes not to have to post back on this topic ;-)