Yahoo SMTP Error 554 Message not allowed - [320]
Caused by incorrect time on sending SMTP client (PC). Mail was relayed thru an SMTP auth postfix server which had correct time. Yahoo is scanning all of the SMTP headers, not just the host that is sending the email inbound to their MX.
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554 Message not allowed
The computer's date was set a week in the future. It looks like the Yahoo servers check timestamps and if they don't reconcile they bounce the message.