Invalid or expired SS certificate of att.yahoo.com:995

RE: Invalid or expired SS certificate of att.yahoo.com:995

My Thunderbird email client pops up a notification of invalid or expired security certificate of att.yahoo email server.  Pointed to server location:  pop.att.yahoo.com:995

I view the error detail that says certificate expired on 12/02/2016

Do anyone know what happened?   Should I permanently store this exception?

Thanl you.

Stephen