enforce STARTTLS and verify MX certificates
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, either.
Opportunistic STARTTLS is defeated with MITM attacks by simply 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?
MTA-STS is NOT a solution. Outbound MTA which don’t talk STARTTLS to their peers would also omit to check for an
This is nonsense from Google and Microsoft at the IETF, as usual.
Using an additional protocol (namely HTTP itself protected by PKIX) for securing SMTP does not enforce ESMTPS, still, let alone certificate validation.
Go through the burden of implementing and enabling 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 pass through your network pipes.
MX (inbound MTA):
530 5.7.0 Must issue a STARTTLS command first (see https://nethence.com/smtp/)
Recipient's Mail eXchanger does not offer STARTTLS capability (see https://nethence.com/smtp/)
And as an email user, you can mention that your email is secured by referring to this page, for example as firstname.lastname@example.org (STARTTLS). You can also tune your email signature exempli gratia as follows. Don’t forget the empty space after the signature dashes:
-- (casual signature) SMTP Health Campaign: enforce STARTTLS and verify MX certificates <https://nethence.com/smtp/>
THE SAD STATE OF SMTP ENCRYPTION https://blog.filippo.io/the-sad-state-of-smtp-encryption/
A system for ensuring & authenticating STARTTLS encryption between mail servers https://github.com/EFForg/starttls-everywhere
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/