Beginning on Feb 2, at around 12:30 PM Eastern Time, my cell phone (Samsung Galaxy s6, my daughter's cell phone, and my desktop computer stopped receiving yahoo verizon mail via our client programs. Nothing was changed on our end in our settings. My laptop continues to work fine with yahoo verizon. Webmail works fine, too. The message our client gives us is that the certificate is not valid. Why should this have stopped working on some devices, but not on others? My wife uses "plain" verizon email and is unaffected; this seems to be only on some devices with the yahoo verizon servers. Outgoing servers seem fine with all devices.