This error indicates (per RFC 8461 Section 4.1) that the MX record name for the MTA-STS policy of an MX host DOES NOT match one or more of the policy's "mx" fields. If the MX record name and the "mx" fields do not match, the MTA-STS policy is invalid.

How can I resolve this issue?

Confirm that the MX record name and the "mx" fields match
If they do not match, update as needed

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MTA-STS MX Host Validation

More Information About Mta-Sts Mx Host Validation

DMARC is the key to improving Email Deliverability!

MxToolbox Delivery Center gives you:

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