Windows Live email issue

I have been using Windows 10 Windows Live email for several weeks on my new Dell desktop computer.  Starting two days ago, every time I open Windows Live, I get an internet security warning window pop-up that says: "the server you are connected to is using a security certificate that could not be verified" "Do you want to continue using the server?"  If I click "Yes", I start receiving messages, but quite often I get another error message showing a server error code 0x800CCC90 - "problem retrieving message" and "unable to send or receive messages for the Att (cmpember) acct".  I have changed nothing since the initial email setup, and everything had been working fine to this point.  I can eventually get all my emails by closing and re-opening Windows Live, but sometimes it takes two to three tries to get all emails downloaded.  My email servers are the standard pop.att.yahoo.com and smtp.att.yahoo.com.  I have called At&t, and they say the security certificate issue can be fixed either by MS or Yahoo.  I have no problem getting my email from the yahoo website.