This error (per RFC 8461 Section 3.3) indicates we were unable to find or connect to the MTA-STS Policy for the domain provided. This test attempts to establish a connection to the MTA-STS Policy. This policy is required to be hosted as a text file on a subdomain with the standard format of mta-sts.domain.com. With that format, your domain would replace the domain.com part of the subdomain.
How can I resolve this issue?
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Set up an MTA-STS Policy by creating a subdomain in the format of mta-sts.domain.com
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If you have already setup the page and are getting the error, verify:
1. The page uses HTTPS by default, as HTTP will be treated as invalid
2. Verify you are not blocking traffic to that subdomain or that you have a misconfiguration on the page