Email Delivery-Security Posture of Email Providers (MTA-STS, 2026)

A reproducible point-in-time dataset (2026-07-18) of email delivery-security signals for 16 email providers: HTTPS, HSTS, published MTA-STS policy (RFC 8461) and its mode (enforce/testing), and RFC 9116 security.txt. Measured via public HTTPS endpoints; fully reproducible; no fabricated values. Records published policy, not proven enforcement; does not check DNS TXT/DNSSEC/DANE. Compiled by Secure-OS. Related reading: best secure email and how to encrypt your email.