Advanced Email Authentication: Beyond the Basics

Once you've implemented SPF, DKIM, and DMARC, there are additional technologies that can further secure your email infrastructure. This guide covers the advanced protocols that complete your email security posture.

Prerequisites: This guide assumes you've already configured SPF, DKIM, and DMARC. If not, start with our Email Security Basics guide first.

MTA-STS: Enforcing Encryption

MTA-STS (Mail Transfer Agent Strict Transport Security) ensures that emails to your domain are always encrypted during transit. Without it, attackers could perform downgrade attacks, forcing email to be sent unencrypted.

How MTA-STS Works

  1. You publish a DNS record announcing MTA-STS support
  2. You host a policy file at https://mta-sts.yourdomain.com/.well-known/mta-sts.txt
  3. Sending servers check your policy before delivering email
  4. If your policy says "enforce," they only deliver over encrypted connections

MTA-STS DNS Record

_mta-sts.example.com. IN TXT "v=STSv1; id=20260123T120000"

MTA-STS Policy File

version: STSv1
mode: enforce
mx: mail.example.com
mx: *.mail.example.com
max_age: 86400
Start with Testing Mode: Set mode: testing initially. This allows you to receive reports without breaking email delivery. Move to enforce once you're confident your mail servers support TLS properly.

TLS-RPT: Encryption Failure Reports

TLS-RPT (TLS Reporting) works alongside MTA-STS to send you reports when other servers have trouble encrypting email to you. It's like getting an error log for email encryption.

TLS-RPT DNS Record

_smtp._tls.example.com. IN TXT "v=TLSRPTv1; rua=mailto:tls-reports@example.com"

Reports include information about:

  • Failed TLS negotiations
  • Certificate validation failures
  • MTA-STS policy fetch failures
  • DANE validation problems

DANE: Certificate Pinning via DNS

DANE (DNS-based Authentication of Named Entities) allows you to specify exactly which TLS certificate should be used for your mail server. This prevents attackers from using fraudulent certificates, even from compromised Certificate Authorities.

How DANE Works

  1. You publish TLSA records in DNSSEC-signed DNS
  2. These records contain cryptographic information about your certificate
  3. Sending servers verify your certificate matches what's in DNS
  4. Even a valid CA-issued certificate will be rejected if it doesn't match

DANE TLSA Record

_25._tcp.mail.example.com. IN TLSA 3 1 1 abc123def456...
DNSSEC Required: DANE only works if your domain uses DNSSEC. Without DNSSEC, attackers could simply modify your DANE records. Implementing DNSSEC is a significant undertaking but provides the foundation for DANE.

BIMI: Brand Logos in Email

BIMI (Brand Indicators for Message Identification) displays your brand logo next to authenticated emails in supporting email clients. It's the reward for good email hygiene.

BIMI Requirements

  • DMARC at p=quarantine or p=reject - Must have enforcement enabled
  • Logo in SVG Tiny PS format - Special format required
  • VMC Certificate (optional) - Verified Mark Certificate for full support

BIMI DNS Record

default._bimi.example.com. IN TXT "v=BIMI1; l=https://example.com/logo.svg; a=https://example.com/vmc.pem"
Brand Trust: BIMI increases email open rates by 10-30% according to studies. Seeing your familiar logo helps recipients trust that the email is genuinely from you.

Implementation Priority Matrix

Here's how to prioritize these advanced features:

Technology Priority Difficulty Prerequisites
MTA-STS High Easy HTTPS subdomain, TLS-capable MX
TLS-RPT High Very Easy Email address for reports
BIMI Medium Medium DMARC enforcement, SVG logo
DANE Lower Hard DNSSEC, certificate management

Security Checklist

Use this checklist to track your email security implementation:

Essential (Do First)

  • SPF record with all sending sources
  • DKIM signing enabled
  • DMARC at p=reject

Important (Do Next)

  • MTA-STS in enforce mode
  • TLS-RPT reporting enabled

Advanced (Nice to Have)

  • BIMI with brand logo
  • DANE with DNSSEC

Test Your Configuration

Check how your domain scores on all these security measures:

Check Your Email Security →

Related Articles

← Email Security Basics Email Deliverability Guide →