MTA-STS max_age is outside the sensible range
MINORWhat it means
max_age is the offending value. Too short and senders re-fetch constantly while caching too little to protect you; above the one-year ceiling the policy is invalid.
How to fix it
Use 604800 (one week) while testing and 1209600 (two weeks) or more once enforcing. The maximum permitted is 31557600.
Check your own domain
This page describes the condition. To find out whether your domain has it, run the MTASTS test or the full checkup, which examines every record at once.
Understand it properly
This page covers one condition. For the whole picture, MTA-STS, defined, or start at the Health Library.
Where this comes from
We judge this against RFC 8461 section 3.2. Every finding on this site cites the specification behind it so you can check the work rather than take our word for it.
Other MTASTS conditions
MTASTS_MAXAGE_INVALID
Every fix on this site is yours to implement. If you would rather someone did it, I take this work directly.