MTA-STS (Mail Transfer Agent Strict Transport Security) is a security standard designed to ensure secure email communication using the SMTP (Simple Mail Transfer Protocol). The goal of this standard is to prevent attackers from intercepting or tampering with emails during their transmission between servers. MTA-STS specifies how servers should validate TLS certificates and establish secure connections.
How MTA-STS Works
MTA-STS operates by publishing specific policies via DNS and HTTPS. These policies define how email servers should be configured for secure email transmission. The standard involves the following steps:
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Publishing the Policy: A domain publishes an MTA-STS policy through a DNS record and an HTTPS file. The DNS TXT record indicates the availability and version of the MTA-STS policy. The HTTPS file, typically located at
https://mta-sts.<domain>/mta-sts.txt, contains detailed information about the policy. -
Policy Validation: When attempting to deliver an email, the sending server's MTA checks the recipient's DNS record to see if MTA-STS is supported. If it is, the MTA downloads and verifies the HTTPS policy file.
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Securing the Connection: Based on the downloaded policy, the sending server's MTA attempts to establish a secure connection with the recipient's server using TLS. If a secure connection cannot be established as per the policy, the email is not delivered.
Configuring MTA-STS
Step 1: Creating and Publishing a DNS Record
A domain must publish a DNS TXT record as follows:
_mta-sts.<domain> IN TXT "v=STSv1; id=<unique-id>"
Where v=STSv1 specifies the version of the standard, and id=<unique-id> is a unique policy identifier that changes with each policy update.
Step 2: Creating the MTA-STS Policy
The policy is a text file that contains the TLS requirements. A typical policy file might look like this:
version: STSv1
mode: enforce
mx: mx1.example.com
mx: mx2.example.com
max_age: 86400
Step 3: Publishing the MTA-STS Policy
The policy must be publicly accessible at https://mta-sts.<domain>/mta-sts.txt. The web server hosting this policy must support HTTPS and have a valid TLS certificate.
Benefits of Implementing MTA-STS
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Enhanced Email Communication Security: MTA-STS protects against MITM (man-in-the-middle) attacks, where an attacker might intercept or alter the content of an email.
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Increased Domain Trustworthiness: Using security standards like MTA-STS increases your domain's trustworthiness and can improve email delivery reputation.
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Simple Management: Once MTA-STS is correctly configured, its maintenance is minimal and does not require regular intervention.
MTA-STS is an important step towards ensuring secure email communication. By implementing this standard, organizations can significantly reduce the risk of email interception or tampering during transmission. This article has provided an overview of the principles, implementation steps, and benefits of MTA-STS, which should be considered when securing email services.
