MTA-STS

MTA-STS guides and tutorials: secure email transport with mandatory TLS encryption.

9 articles

March 2026

The 4 email security tools managed by CaptainDNS: MTA-STS, BIMI, TLS-RPT and DMARC

CaptainDNS · March 13, 2026

Email Security: 4 Free Managed Tools to Protect Your Domains

CaptainDNS offers four free tools to secure your emails: MTA-STS hosting, BIMI hosting, TLS-RPT monitoring and DMARC monitoring. No infrastructure to manage.

Diagram showing the progression of MTA-STS deployment from testing mode to enforce mode

CaptainDNS · March 10, 2026

From testing to enforce: a progressive MTA-STS deployment strategy

Testing mode monitors, enforce mode blocks. This guide details the four steps to deploy MTA-STS progressively, from the initial policy to TLS-RPT reports, through to switching to enforce.

Comparative diagram of MTA-STS and DANE protocols for email transport security

CaptainDNS · March 10, 2026

MTA-STS vs DANE: which protocol to choose for securing email transport?

MTA-STS uses HTTPS, DANE relies on DNSSEC. Both enforce mandatory TLS encryption between email servers, but they deploy differently. Complete comparison to make the right choice.

Diagram of an SMTP downgrade attack showing an attacker intercepting the STARTTLS connection between two email servers

CaptainDNS · March 9, 2026

SMTP Downgrade Attacks: How They Work and How to Defend Against Them

STARTTLS encrypts your emails, but a network attacker can strip that protection away. Learn how SMTP downgrade attacks work and what solutions block them.

February 2026

Analyzing a TLS-RPT report: JSON structure and TLS email error types

CaptainDNS · February 14, 2026

Analyze and leverage your TLS-RPT reports: detect issues before they impact your emails

A practical guide to reading and leveraging TLS-RPT reports: JSON structure, failure types, diagnosing certificate and STARTTLS errors, and a daily analysis workflow.

TLS-RPT: monitoring TLS encryption failures in email delivery

CaptainDNS · February 10, 2026

TLS-RPT: The Complete Guide to Monitoring Email TLS Security

TLS-RPT (RFC 8460) lets you receive reports on TLS encryption failures in your email delivery. Learn how to configure it, read reports, and take action.

MTA-STS troubleshooting: diagnosing and resolving common errors

CaptainDNS · February 9, 2026

MTA-STS Not Working? Complete Troubleshooting Guide

Your MTA-STS policy is unreachable, the TLS certificate is rejected, or enforce mode is blocking emails? This troubleshooting guide covers common errors and their solutions.

Configure MTA-STS for Microsoft 365, Google Workspace, and Cloudflare

CaptainDNS · February 8, 2026

Configure MTA-STS for Microsoft 365 and Google Workspace

Step-by-step tutorial to deploy MTA-STS on Microsoft 365, Google Workspace, and Cloudflare. MX patterns, policy file, hosting, and validation included.

MTA-STS: securing email transport with mandatory TLS encryption

CaptainDNS · February 6, 2026

MTA-STS: the complete guide to securing your email transport

MTA-STS (RFC 8461) enforces TLS encryption for incoming emails. Learn how to configure it, choose between testing and enforce modes, and validate your deployment.