Opportunistic TLS can be downgraded by an attacker in the network path. MTA-STS closes that hole and is increasingly a checkbox in security questionnaires.
How to implement MTA-STS#
- Publish a _mta-sts TXT record with an id value you will change on each policy update.
- Host a policy file at https://mta-sts.yourdomain.com/.well-known/mta-sts.txt listing your MX hosts.
- Start in mode: testing, then move to mode: enforce after reviewing TLS-RPT reports.
- Publish _smtp._tls TXT with rua=mailto: to receive TLS-RPT reports.
version: STSv1
mode: enforce
mx: aspmx.l.google.com
mx: *.googlemail.com
max_age: 604800How to verify it worked#
Send a test message to seed mailboxes at Gmail, Outlook, and Yahoo, then inspect the Authentication-Results and delivery headers. Repeat after any DNS or sending-platform change.
Common mistakes#
- Listing MX hostnames that do not exactly match certificate names.
- Forgetting to bump the id when the policy file changes, so caches never refresh.
Frequently asked questions#
Does MTA-STS affect outbound mail?
Only when the recipient domain publishes a policy. Your own policy protects mail coming to you.
Is DANE better than MTA-STS?
DANE requires DNSSEC and is stronger, but MTA-STS is easier to adopt. Microsoft and Google both support MTA-STS.