MTA-STS Policy Audit

Audit an MTA-STS policy against the MX hosts it is meant to authorise. Reports an MX host no pattern matches, which under enforce mode refuses every message to that host; testing or none mode, where nothing is enforced; a max_age under a day, where a receiver that cannot fetch the policy stops honouring it almost immediately; and a DNS record with no id, without which a cached policy is never refreshed.