Thanks for all the replies to my original message. It confirmed to me that the problem is sourced here. Just for everyone's information, I am a retired senior systems analyst with 40 years of experience in every aspect of computers and computing.
Briefly, your replies confirmed to me that it is my e-mail client program that is not handling Yahoo's security certificate properly. BTW -- Yahoo is the only Internet server I've ever received this warning from. This has nothing whatsoever to do with a virus or malware. The messages go right through my anti-virus program, as they should. The warning states that the security certificate is valid, but was issued by a different entity than that for which the certificate covers. Symantic is the issuer, on behalf of Yahoo. Apparently Yahoo farmed the certificate handling out to a third party - Symantic - and that's the cause of the warning.
Most everyone these days sends and retrieves their e-mail as "Webmail" through their Internet browser. I do not. E-mail is transmitted and received on my system in a completely different form than Webmail -- it is SMTP/POP messages and requires a client program (not a browser) to handle messages. My client program is rather old, and apparently is not designed to handle third-party security certificates issuances.
Thanks to everyone who replied.