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MTA-STS and TLS-RPT in 2026: Enforcing Strict Email Transit Encryption

Standard SMTP opportunistic TLS (STARTTLS) is vulnerable to downgrade attacks. Here is how to configure MTA-STS policy files and TLS-RPT records for enterprise email transit security.

Tayyab MughalFounder & AI Chief1 min read

The vulnerability of opportunistic STARTTLS

Traditional email transfer uses opportunistic TLS: if an attacker intercepts the connection (MITM) and strips the STARTTLS command, mail servers silently fall back to sending emails in plain unencrypted text.

MTA-STS (RFC 8461) closes this vulnerability by publishing an authoritative HTTPS policy requiring sending mail transfer agents (MTAs) to enforce TLS with valid certificates.

1. Publishing the MTA-STS Policy File

Host a plain-text file at https://mta-sts.yourdomain.com/.well-known/mta-sts.txt:

TEXT
version: STSv1
mode: enforce
mx: feedback-smtp.us-east-1.amazonses.com
mx: *.sadasend.com
max_age: 604800

2. Configuring DNS Records

Add the corresponding DNS TXT records to announce MTA-STS and receive automated TLS-RPT failure reports from Google and Microsoft.

DNS
; MTA-STS Announcement Record
_mta-sts.yourdomain.com.  IN TXT  "v=STSv1; id=2026082501;"

; TLS-RPT Aggregate Reporting Record
_smtp._tls.yourdomain.com. IN TXT "v=TLSRPTv1; rua=mailto:tls-reports@sadasend.com;"
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Tayyab MughalFounder & AI Chief

Building SadaSend — transactional email with an MCP server that has a ceiling. Writes about deliverability, email infrastructure, and what happens when you hand an autonomous agent a sending credential.

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