'Protocol error' when sending using OpenSMTPD as a relay

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Earlier this month I started receiving protocol errors such as the
following when attempting to send via my OpenSMTPD relay.

    Starting delivery, 1 message(s) in queue.
    Starting delivery: host: mail.domain.com protocol: smtp file: 1572382004.12919
    From: <[email protected]> to: <[email protected]>
    Message-Id: <[email protected]>
    smtp: Failed: 
    Sending failed: Protocol error
    Delivery complete, 1 message(s) remain.

My configuration looks like the following:

    mail.domain.com smtp starttls insecure user=relay pass=secret

I'm attaching the protocol log as seen from the perspective of
OpenSMTPD. As far as I can tell nullmailer is just quitting as soon as
TLS is established. What's strange is that it's worked before, and I'm
not sure what may have caused it to stop working.

Thanks,

Robert Gill

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Oct 29 21:53:08 mail smtpd[12026]: 437f726d5b895907 smtp event=connected address=192.0.2.100 host=nullmailer.domain.com
Oct 29 21:53:08 mail smtpd[12026]: smtp: 0x1329f90: >>> 220 domain.com ESMTP OpenSMTPD
Oct 29 21:53:08 mail smtpd[12026]: smtp: 0x1329f90: <<< EHLO nullmailer.domain.com
Oct 29 21:53:08 mail smtpd[12026]: smtp: 0x1329f90: STATE_CONNECTED -> STATE_HELO
Oct 29 21:53:08 mail smtpd[12026]: smtp: 0x1329f90: >>> 250-domain.com Hello nullmailer.domain.com [192.0.2.100], pleased to meet you
Oct 29 21:53:08 mail smtpd[12026]: smtp: 0x1329f90: >>> 250-8BITMIME
Oct 29 21:53:08 mail smtpd[12026]: smtp: 0x1329f90: >>> 250-ENHANCEDSTATUSCODES
Oct 29 21:53:08 mail smtpd[12026]: smtp: 0x1329f90: >>> 250-SIZE 36700160
Oct 29 21:53:08 mail smtpd[12026]: smtp: 0x1329f90: >>> 250-DSN
Oct 29 21:53:08 mail smtpd[12026]: smtp: 0x1329f90: >>> 250-STARTTLS
Oct 29 21:53:08 mail smtpd[12026]: smtp: 0x1329f90: >>> 250 HELP
Oct 29 21:53:08 mail smtpd[12026]: smtp: 0x1329f90: <<< STARTTLS
Oct 29 21:53:08 mail smtpd[12026]: smtp: 0x1329f90: >>> 220 2.0.0: Ready to start TLS
Oct 29 21:53:08 mail smtpd[12026]: smtp: 0x1329f90: STATE_HELO -> STATE_TLS
Oct 29 21:53:09 mail smtpd[12026]: 437f726d5b895907 smtp event=starttls address=192.0.2.100 host=nullmailer.domain.com
Oct 29 21:53:09 mail smtpd[12026]: smtp: 0x1329f90: STATE_TLS -> STATE_HELO
Oct 29 21:53:09 mail smtpd[12026]: smtp: 0x1329f90: <<< EHLO nullmailer.domain.com
Oct 29 21:53:09 mail smtpd[12026]: smtp: 0x1329f90: STATE_HELO -> STATE_HELO
Oct 29 21:53:09 mail smtpd[12026]: smtp: 0x1329f90: >>> 250-domain.com Hello nullmailer.domain.com [192.0.2.100], pleased to meet you
Oct 29 21:53:09 mail smtpd[12026]: smtp: 0x1329f90: >>> 250-8BITMIME
Oct 29 21:53:09 mail smtpd[12026]: smtp: 0x1329f90: >>> 250-ENHANCEDSTATUSCODES
Oct 29 21:53:09 mail smtpd[12026]: smtp: 0x1329f90: >>> 250-SIZE 36700160
Oct 29 21:53:09 mail smtpd[12026]: smtp: 0x1329f90: >>> 250-DSN
Oct 29 21:53:09 mail smtpd[12026]: smtp: 0x1329f90: >>> 250-AUTH PLAIN LOGIN
Oct 29 21:53:09 mail smtpd[12026]: smtp: 0x1329f90: >>> 250 HELP
Oct 29 21:53:09 mail smtpd[12026]: smtp: 0x1329f90: <<< QUIT
Oct 29 21:53:09 mail smtpd[12026]: smtp: 0x1329f90: >>> 221 2.0.0: Bye
Oct 29 21:53:09 mail smtpd[12026]: smtp: 0x1329f90: STATE_HELO -> STATE_QUIT
Oct 29 21:53:09 mail smtpd[12026]: 437f726d5b895907 smtp event=closed address=192.0.2.100 host=nullmailer.domain.com


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