Does MTA-STS rely on a specific DNS record name for discovery?

Yes, absolutely. MTA-STS (Mail Transfer Agent Strict Transport Security) fundamentally relies on a specific DNS record for policy discovery. Without this DNS record, sending mail servers would have no way of knowing that your domain supports MTA-STS, and the entire security mechanism wouldn't work.

The core idea behind MTA-STS is to enable domains to declare their ability to receive TLS-encrypted emails and to specify which mail servers are authorized to receive mail for them. This helps prevent man-in-the-middle (MITM) attacks where an attacker could downgrade a connection to unencrypted SMTP or redirect email to a malicious server.

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The MTA-STS protocol is implemented by having a DNS record that specifies that a mail server can fetch a policy file from a defined subdomain.

But for this system to function, there needs to be a standardized discovery mechanism. That's where DNS comes in. The protocol specifies a particular DNS record that all compliant sending MTAs know to look for.

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MTA-STS uses a TXT record at a specially designated subdomain. For a domain like example.com, the MTA-STS DNS record must be published at _mta-sts.example.com.

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You must then add the necessary DNS TXT record so that sending MTAs can discover and retrieve your policy.

This TXT record is not the policy itself. Instead, it serves two primary functions:

• Signal support: Its mere presence signals to sending servers that you have an MTA-STS policy.
• Provide policy details: The record contains a version tag (v=STSv1) and an ID, which helps servers know if your policy has been updated.

As explained in this guide, when a sending server wants to deliver an email, it performs a DNS lookup for this specific _mta-sts record. If found, the sender knows to fetch the full policy from a predefined HTTPS endpoint and apply its rules.

The discovery process is a chain of events. The DNS record is the first link in that chain.

• A sending server looks up the MX records for your domain to find your mail servers.
• Simultaneously, it checks for the existence of the _mta-sts.yourdomain.com TXT record.
• If the record exists, the sender fetches the policy file from a specific URL, which is https://mta-sts.yourdomain.com/.well-known/mta-sts.txt.
• The sender validates the policy file (checking the certificate and matching the MX records from the policy to its own lookup) and caches it.
• For subsequent emails, the sender enforces the cached policy, ensuring connections are encrypted and sent only to authorized servers.

While effective, this reliance on DNS isn't without potential issues. The primary concern is that MTA-STS relies on DNS TXT records for policy discovery without mandating DNSSEC. DNSSEC is a technology that authenticates DNS responses, protecting against DNS spoofing.

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MTA -STS does not mandate the use of DNSSEC and despite hosting the policy on a HTTPS secured server, still relies on a traditional DNS lookup.

Because DNSSEC is not required, an attacker with control over the network path could potentially block the DNS query for the MTA-STS record. If the sending server can't find the record, it will assume no policy exists and may proceed with an insecure connection, negating the protection MTA-STS offers. This is known as a downgrade attack.

In conclusion, the _mta-sts DNS record is not just a part of the MTA-STS standard, it is the essential starting point. Its specific name and location are what allow the protocol to be discovered and adopted in a standardized way across the entire email ecosystem.