MTA-STS record
The TXT record at _mta-sts followed by the domain, v=STSv1 plus an id, telling senders a policy exists and when it has changed.
What it means
The record has two required fields: v=STSv1 first, and id=, an identifier of 1 to 32 letters and digits. Senders compare the id with the one they cached; a new value tells them to fetch the policy file again. The RFC's example uses a timestamp; ids have no order, only change.
The record does not hold the policy itself, which lives on the HTTPS policy host. Forgetting to change the id after editing the policy means senders keep using the cached copy until it expires.
If more than one record starting with v=STSv1 is found, senders treat the domain as having no policy.
Example
_mta-sts.example.com. IN TXT "v=STSv1; id=20261021120000Z;"Related terms
On this site
Sources
What this definition rests on. The wording is ours; the facts are theirs.
All 432 terms in the glossaryThe wiki: blocklists and bounce codes