Mail delivery failure due to GnuTLS X.509 validation

mail_gnuttls_x509_fail.md

I quite commonly see undelivered mail to bounce@tweet.twitter.com on the outgoing mail queue. (DSNs and out-of-office replies with empty senders - so not critical mail.)

(The Twitter mail is recieved by en25.com/eloqua.net which is infrastructure for "Oracle Eloqua Marketing Cloud".)

This isn’t a new issue, but I thought I’d drop a note in public about it.

If I run exim4 -v -M on a delivery attempt I can see that it disconnects immediately after attempting to establish a STARTTLS session, so falls back to retrying without TLS.

  SMTP<< 220 P01SNJ018.eloqua.net Microsoft ESMTP MAIL Service, Version: 10.0.14393.4169 ready at  Tue, 10 Jan 2023 05:50:33 -0500
  SMTP>> EHLO foo.example.com
  SMTP<< 250-P01SNJ018.eloqua.net Hello [10.32.120.102]
         250-TURN
         250-SIZE 2097152
         250-ETRN
         250-PIPELINING
         250-DSN
         250-ENHANCEDSTATUSCODES
         250-8bitmime
         250-BINARYMIME
         250-CHUNKING
         250-VRFY
         250-TLS
         250-STARTTLS
         250 OK
  SMTP>> STARTTLS
  SMTP<< 220 2.0.0 SMTP server ready
  SMTP(close)>>
  TLS session: (gnutls_handshake): Key usage violation in certificate has been detected.: delivering unencrypted to H=mail.en25.com [209.167.231.14] (not in hosts_require_tls)

But, the server won't accept a bounce on an unencrypted connection.

  SMTP<< 220 P01SNJ015.eloqua.net Microsoft ESMTP MAIL Service, Version: 10.0.14393.4169 ready at  Tue, 10 Jan 2023 05:50:34 -0500
  SMTP>> EHLO foo.example.com
  SMTP<< 250-P01SNJ015.eloqua.net Hello [10.32.120.104]
         250-TURN
         250-SIZE 2097152
         250-ETRN
         250-PIPELINING
         250-DSN
         250-ENHANCEDSTATUSCODES
         250-8bitmime
         250-BINARYMIME
         250-CHUNKING
         250-VRFY
         250-TLS
         250-STARTTLS
         250 OK
  SMTP>> MAIL FROM:<> SIZE=2451
  SMTP>> RCPT TO:<bounce@tweet.twitter.com>
  SMTP>> BDAT 2092 LAST
  SMTP<< 530 5.7.0 Must issue a STARTTLS command first
  SMTP<< 530 5.7.0 Must issue a STARTTLS command first
  SMTP>> QUIT
  SMTP(close)>>

The mailserver is the Ubuntu 20.04 exim4-daemon-heavy (4.93-13ubuntu1.7) using libgnutls (3.6.13-2ubuntu1.7)

I can see using TLS debug tools that the certificate isn't signed. But certificate errors aren't uncommon for mail servers, and it doesn't usually impede delivery.

echo QUIT | gnutls-cli --starttls-proto=smtp -p 25 --no-ca-verification mail.en25.com

puts the following in stderr

|<1>| Peer's certificate does not allow digital signatures. Key usage violation detected.
*** Fatal error: Key usage violation in certificate has been detected.

If I pull out the X.509 Key Usage section

echo QUIT | gnutls-cli --starttls-proto=smtp -p 25 --no-ca-verification --insecure --save-cert=/dev/stdout mail.en25.com | certtool -i

   Key Purpose (not critical):
      TLS WWW Server.
   Key Usage (critical):
      Key encipherment.
      Data encipherment.

Checking against servers I know will accept STARTTLS, such as smtp.google.com

   Key Usage (critical):
        Digital signature.
   Key Purpose (not critical):
       TLS WWW Server.

and a server with a standard letsencrypt cert

   Key Usage (critical):
      Digital signature.
      Key encipherment.
   Key Purpose (not critical):
      TLS WWW Server.
      TLS WWW Client.

I can see that they have "Digital signature" in the X.509 key usage section.

Given other servers, eg ones using OpenSSL, will establish a connection regardless, is the hard requirement for "Digital signature" in the key usage the correct behaviour?

And, given plaintext fallback, is there any way to switch this off without recompiling?