Yahoo certificate for SMTP server expires on a regular basis. What to do?

Client called to say that they were unable to send email. They are using Mail on MacOS X 10.9.5
I quit the mail program, started again, and got a notice that the Yahoo certificate for Yahoo's SMTP server had expired.  I allowed the exception, and the mail went out.

Client says outgoing mail stops regularly.

Is there anything wrong with Yahoo or the Mac Mail program?
Anything I can do?

Thanks.
computerlarryAsked:
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btanExec ConsultantCommented:
you probably can from the client check the SMTP server if it causing any issue to client connecting to it since previously it is fine with the port and SSL enabled. E.g. for the case of smtp.mail.yahoo.com, if it alright then you should see this
The mailservers of smtp.mail.yahoo.com can be reached through a secure connection.
https://ssl-tools.net/mailservers/smtp.mail.yahoo.com
So if the yahoo SMTP server is ok then it is the client itself ...

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gheistCommented:
No need to worry, they renewed certificate 3 days ago. SMTP SSL certificate validation should be optional, so fault is yours, not yahoos.
btanExec ConsultantCommented:
Do check again w/o exception (i.e. turn off the insecure authentication for the exception and enable SSL) the email should work already http://osxdaily.com/2014/10/28/fix-mail-smtp-sending-errors-os-x-mac/
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