SMTP Health Campaign

Enforce STARTTLS and verify MX certificates

Dear postmasters

Email is dying, and for a good reason: SMTP is kept insecure all around the world. As of 2021, the large majority of Mail eXchangers are still not enforcing STARTTLS nor providing a valid DANE or PKIX certificate. And even those few outbound MTAs which do enforce STARTTLS, mostly won’t verify the certificate on the other end.

Opportunistic STARTTLS is defeated by MITM attacks downgrading to plain-text, and the horrific trust-any default policy gets defeated by any self-signed or private certificate in the middle.

Let us enforce inbound / outbound STARTTLS and Mail eXanger certificate verification, shall we?

KISS

MTA-STS is NOT a solution. Outbound MTA which don’t talk STARTTLS to their peers would also omit to check for an _mta-sts record. This is nonsense from Google and Microsoft at the IETF, just like for OAuth2. Using an additional protocol (namely HTTP itself protected by PKIX) for securing SMTP does not enforce ESMTPS, let alone certificate validation.

Enable MTA-STS if you wish, but unless you make STARTTLS and a valid trust chain mandatory at the SMTP level, some clear-text messages will still pass through your network pipes.

New practice

Inbound MX

• Enforce STARTTLS
• Deploy valid server certificate(s) with either PKIX or DANE (possibly multiple certificates)
• Allow TLS v1.0 onwards and provide an RSA certificate for backward compatibility

• Eventually tune the error message: 530 5.7.0 Must issue a STARTTLS command first (https://nethence.com/smtp/)
• Eventually setup a non-compliant and renegate MTA-STS DNS record: _mta-sts IN TXT "STARTTLS is enforced anyhow (https://nethence.com/smtp/)"

Outbound relay

• Enforce STARTTLS
• Make sure your PKIX CA trust store is well defined and kept up-to-date
• Enforce PKIX and DANE for MX certificate verification
• Allow TLS v1.0 onwards for backward compatibility
• Eventually tune the bounce error message: Recipient's Mail eXchanger does not offer STARTTLS capability (https://nethence.com/smtp/)

Public network survey

• Jan 2020 (last year)
• Jan 2021 (on-going…)

Resources

THE SAD STATE OF SMTP ENCRYPTION https://blog.filippo.io/the-sad-state-of-smtp-encryption/

POSTFIX AND STARTTLS https://pub.nethence.com/mail/postfix-tls

Let’s tune our cipher suites https://pub.nethence.com/security/ciphers

Strong Ciphers (see Other Software section) https://syslink.pl/cipherlist/

Applied Crypto Hardening https://bettercrypto.org/