Henry Jensen (2019-12-04 23:08 +0100):
> $ openssl s_client -connect 192.168.56.121:25 -starttls smtp
...
> I did verify, that this attack worked on my unpatched OpenBSD 6.6 Box.
> But I didn't get much further. After the authentication succeeded
> I continued with MAIL FROM: and RCPT TO: After the RCPT TO: the
> connection was aborted.

That is openssl s_client getting in your way. From the man page:

     When used interactively (which means neither -quiet nor -ign_eof have
     been given), the session will be renegotiated if the line begins with an
     R; if the line begins with a Q or if end of file is reached, the
     connection will be closed down.

The workaround is to use lowercase commands.

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