Boost Your Email Security: How SMTP DANE Complements SPF, DKIM, and DMARC

Arjan Cornelissen
Exchange, Office, Office-365, Security
Page content

Why SMTP DANE Is a Powerful Addition to Email Security

Email remains one of the most critical communication tools for businesses, but it’s also a frequent target for cyberattacks. To combat spoofing, phishing, and impersonation, many organizations already rely on SPF, DKIM, and DMARC. These protocols help verify the sender’s identity and ensure message integrity. However, they don’t fully protect the transport layer—the actual path your email takes across the internet.

That’s where SMTP DANE (DNS-based Authentication of Named Entities) comes in. It adds a crucial layer of security by ensuring that emails are encrypted and delivered only to the intended mail servers, verified through DNSSEC.

What Is SMTP DANE?

SMTP DANE is a protocol that enhances the security of email transmission by using DNSSEC and TLSA records to authenticate mail servers. It ensures that:

  • Emails are encrypted using TLS.
  • The receiving server’s identity is verified before transmission.
  • Downgrade and man-in-the-middle (MITM) attacks are prevented.

Unlike opportunistic TLS, which tries to encrypt but falls back to plaintext if TLS isn’t available, SMTP DANE requires encryption and validates the server’s certificate using DNS-based records.

How It Works

Here’s a simplified breakdown of the SMTP DANE process:

  1. DNS Lookup with DNSSEC: The sending mail server queries DNS for the recipient domain’s MX records. DNSSEC ensures these records haven’t been tampered with.

  2. TLSA Record Retrieval: The server then looks for a TLSA record associated with the MX host. This record contains the expected certificate or public key.

  3. Certificate Validation: The sending server compares the TLS certificate presented by the receiving server with the TLSA record.

  4. Secure Transmission: If the certificate matches, the email is transmitted securely using TLS. If not, the message is not sent, and retries are scheduled.

This process ensures that the email is not only encrypted but also delivered to a verified recipient server.

Why SMTP DANE Matters

While SPF, DKIM, and DMARC focus on sender authentication and message integrity, SMTP DANE addresses a different but equally important aspect: transport security.

Here’s how it complements existing protocols:

Protocol Focus Area What It Secures
SPF Sender IP Prevents spoofing
DKIM Message Signature Ensures message integrity
DMARC Policy Enforcement Aligns SPF/DKIM with the sender domain
SMTP DANE Transport Layer Secures server identity and encryption

By combining these protocols, organizations can achieve end-to-end email security—from verifying the sender to ensuring the message is encrypted and delivered to the right server.

Requirements for SMTP DANE

To implement SMTP DANE, your domain must:

  • Support DNSSEC.

  • Publish TLSA records for your mail servers.

  • Use a mail system that supports DANE validation (e.g., Exchange Online).

Microsoft Exchange Online supports SMTP DANE and will retry delivery attempts for up to 24 hours if validation fails, ensuring secure delivery without compromising reliability 1.

Step-by-Step Guide to Implementing SMTP DANE

Implementing SMTP DANE requires coordination between your DNS provider and your email infrastructure. Here’s how to do it:

✅ Step 1: Enable DNSSEC for Your Domain

DNSSEC is a prerequisite for DANE. You’ll need to:

  • Log in to your domain registrar or DNS hosting provider.

  • Enable DNSSEC signing for your domain.

  • Publish the DS (Delegation Signer) record at your registrar.

Tip: DNSSEC setup varies by provider. Refer to your DNS host’s documentation for specific instructions.

✅ Step 2: Identify Your MX Records

Determine which mail servers handle your domain’s email:

nslookup -type=mx yourdomain.com

✅ Step 3: Retrieve the TLS Certificate from Your Mail Server

You’ll need the certificate or public key of your mail server:

openssl s_client -connect mail.yourdomain.com:25 -starttls smtp

Copy the certificate details from the output.

✅ Step 4: Generate the TLSA Record

Use a tool like HashTLSA or an online TLSA generator. You’ll need:

  • Certificate usage (e.g., 3 for DANE-EE)

  • Selector (e.g., 1 for SPKI)

  • Matching type (e.g., 1 for SHA-256)

  • The hash of the certificate or public key

Example TLSA record:

_25._tcp.mail.yourdomain.com. IN TLSA 3 1 1 <certificate_hash>

✅ Step 5: Publish the TLSA Record in DNS

Add the TLSA record to your DNS zone file under the appropriate subdomain (e.g., _25._tcp.mail.yourdomain.com).

✅ Step 6: Verify the TLSA Record

Use tools like dig or online DANE validators:

dig +dnssec _25._tcp.mail.yourdomain.com TLSA

✅ Step 7: Test SMTP DANE Functionality

Use these tools to validate your setup:

Final Thoughts

SMTP DANE is a powerful addition to your email security toolkit. It fills a critical gap left by SPF, DKIM, and DMARC by securing the transport path of your emails. If your organization values privacy, integrity, and trust in email communications, enabling SMTP DANE is a smart move.

Want to learn more or need help implementing SMTP DANE? Reach out or leave a comment below!

1: How SMTP DANE works - Microsoft Learn