Fix · Cloudflare
Fix “Mail transport security (MTA-STS)” on Cloudflare
Publish _mta-sts TXT and host /.well-known/mta-sts.txt with mode: enforce.
No server-level snippet for this control on Cloudflare; apply the guidance below.
Verify the fix
Re-run the Mail transport security (MTA-STS) check after deploying. Test config changes in staging first and keep a rollback: a wrong header can break a site faster than a missing one.
Other stacks: nginx · Apache · Caddy · Express / Node