The SubCA (Symantec Class 3 Secure Server CA - G4) that signed all of their TLS certificates expired September 1, 2017. This breaks any secure mail delivery that verifies the TLS certificate chain.
#Fail Yahoo... again.
Yahoo.com Mail servers TLS certs are non-Valid
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Re: Yahoo.com Mail servers TLS certs are non-Valid
#2They expired two weeks ago, and it's only just now being noticed?