At Microsoft Ignite 2020, Microsoft announced support for MTA-STS, or Mail Transfer Agent Strict Transport Security. This is covered in RFC 8461 and it includes making TLS for mail flow to your domains mandatory whereas it is currently down to the decision of the sender.
You can publish your SMTP endpoint and offer the STARTTLS verb but there is no requirement for the sender to use it unless you have configured the sender as well to ensure that they only email you over TLS (for example RequireTLS and TLSDomain settings in Exchange Server/Exchange Online connectors). MTA-STS allows you, the domain owner, to publish your TLS requirements.
You publish your requirements by placing a policy file in your websites “.well-known” directory. The policy will have version: STSv1 and mode: [testing|enforce|none] and mx record. “Testing” for mode says send the delivery of the email will work regardless of success or failure, but also send a report if it failed. “Enforce” means security must pass or the message delivery fails and “none” clears the policy, acting as if you don’t have a policy but giving you a route to remove the policy cleanly rather than what might happen if the policy was to disappear (mail flow should stop). The policy will also have a max_age value in seconds on how long the sender should cache the policy. For example:
version: STSv1 mode: testing mx: mail.domain.com mx: c7solutions-com.mail.protection.outlook.com max_age: 86400
In the above example, my policy is for testing and so I have set a short max_age value, though a value of weeks or more would typically be expected with 31557600 being the largest value you can set (a year and 1/4 of a day in seconds).
The text file must be called mta-sts.txt in the .well-known folder of the mts-sts domain, for example https://mta-sts.c7solutions.com/.well-known/mta-sts.txt
Finally, the policy is published via DNS with the _mta-sts subdomain record:
_mta-sts.c7solutions.com TXT "v=STSv1; id=202009241541"
This DNS record must be v=STSv1 and the id needs to be a value that changes when the policy file changes, so I have just used a date string, but it could be anything that you change as the policy changes. The DNS record can also be a CNAME record instead of a TXT record when someone else hosts your email infrastructure and in this case the value points to the MTA-STS domain of the provider instead.
Testing mode was mentioned above, and that is covered in my second blog post today on this topic – Reporting on MTA-STS Failures