This error indicates that there is a syntax problem with your MTA-STS Policy .TXT file hosted on your https://mta-sts.domain.com/.well-known/mta-sts.txt URL. Possible errors include:

No .TXT file found at "/.well-known/mta-sts.txt"
Required key-value pairs are missing (V, Mode, and Max_age are all required keys)
Key is present, but the value is missing or incorrect (typo or wrong value)

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