gmail error? 454 454 TLS not available due to local problem (state 9)

[Rich Ernst]

Would someone be so kind as to explain what this error message from
gmail refers to?  Seems to happen every time I send to him.

He has his own domain through cleverdot.com registrar, forwarding to,
I think, his ATT Uverse (yahoo?) account.  Mail does eventually seem
to get through, just delayed by days.

Is it a Gmail problem, or the recipient's problem?
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454 454 TLS not available due to local problem (state 9)
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It's just a warning that the email I sent will be delayed, so not
terminal, apparently, and I don't get it when sending to anyone else
(haven't sent to anyone else I know of with ATT or yahoo but will).

What does this mean, in layman's terms?

Thanks,

Rich