544 6.6.0
On 3/24/2022 10:07:00 AM Error was, "The message could not be sent to the SMTP server. The transport error code was 0x800ccc6f. The server response was 554 6.6.0." This came from an Excel VBA Macro for Email automation. Which has worked for years. Recreating passwords did not help.
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Yahoo Mail team