Got unknown command for MAIL state: XCLIENT

[murtll]

I am trying to get MailHog working in docker behind Nginx Mail Proxy.
I have successfully configured Nginx to proxy MailHog Web UI and API with TLS termination, but I also want to secure SMTP server with minimal authentication.

I am trying to do it like that:

1. I have MailHog docker container running on server with published 1025 port for SMTP
2. I have very simple auth server running on server on port 9000. It implements Nginx Mail Auth protocol
3. I have Nginx mail configuration here:

mail {
    server_name mailhog.example.com;

    auth_http 127.0.0.1:9000;
    proxy_pass_error_message on;

    starttls on;

    ssl_certificate     /etc/letsencrypt/live/mailhog.example.com/fullchain.pem;
    ssl_certificate_key /etc/letsencrypt/live/mailhog.example.com/privkey.pem;

    server {
        listen    587;
        protocol  smtp;
        smtp_auth login plain cram-md5;
    }
}

Nginx config seems to work ok, my SMTP requests can go through Nginx to the MailHog SMTP, but then I get an 500 Unknown command error

sendmail ouput:

sendmail: recv:'220 mailhog.example.com ESMTP ready'
sendmail: send:'EHLO 1466b377c911'
sendmail: recv:'250-mailhog.example.com'
sendmail: recv:'250-AUTH PLAIN LOGIN CRAM-MD5'
sendmail: recv:'250 STARTTLS'
sendmail: send:'AUTH LOGIN'
sendmail: recv:'334 VXNlcm5hbWU6'
sendmail: send:''
sendmail: recv:'334 UGFzc3dvcmQ6'
sendmail: send:''
sendmail: recv:'500 Unrecognised command'
sendmail:  failed

MailHog logs:

2023/02/22 09:17:08 [SMTP 172.18.0.1:48398] Starting session
2023/02/22 09:17:08 [SMTP 172.18.0.1:48398] [PROTO: INVALID] Started session, switching to ESTABLISH state
2023/02/22 09:17:08 [SMTP 172.18.0.1:48398] Sent 39 bytes: '220 dev.mailhog.liis.su ESMTP MailHog\r\n'
2023/02/22 09:17:08 [SMTP 172.18.0.1:48398] Received 26 bytes: 'EHLO mailhog.example.com\r\n'
2023/02/22 09:17:08 [SMTP 172.18.0.1:48398] [PROTO: ESTABLISH] Processing line: EHLO mailhog.example.com
2023/02/22 09:17:08 [SMTP 172.18.0.1:48398] [PROTO: ESTABLISH] In state 1, got command 'EHLO', args 'mailhog.example.com'
2023/02/22 09:17:08 [SMTP 172.18.0.1:48398] [PROTO: ESTABLISH] In ESTABLISH state
2023/02/22 09:17:08 [SMTP 172.18.0.1:48398] [PROTO: ESTABLISH] Got EHLO command, switching to MAIL state
2023/02/22 09:17:08 [SMTP 172.18.0.1:48398] Sent 31 bytes: '250-Hello mailhog.example.com\r\n'
2023/02/22 09:17:08 [SMTP 172.18.0.1:48398] Sent 16 bytes: '250-PIPELINING\r\n'
2023/02/22 09:17:08 [SMTP 172.18.0.1:48398] Sent 16 bytes: '250 AUTH PLAIN\r\n'
2023/02/22 09:17:08 [SMTP 172.18.0.1:48398] Received 57 bytes: 'XCLIENT ADDR=xxx LOGIN=dev NAME=[UNAVAILABLE]\r\n'
2023/02/22 09:17:08 [SMTP 172.18.0.1:48398] [PROTO: MAIL] Processing line: XCLIENT ADDR=xxx LOGIN=dev NAME=[UNAVAILABLE]
2023/02/22 09:17:08 [SMTP 172.18.0.1:48398] [PROTO: MAIL] In state 6, got command 'XCLIENT', args 'ADDR=xxx LOGIN=dev NAME=[UNAVAILABLE]'
2023/02/22 09:17:08 [SMTP 172.18.0.1:48398] [PROTO: MAIL] In MAIL state
2023/02/22 09:17:08 [SMTP 172.18.0.1:48398] [PROTO: MAIL] Got unknown command for MAIL state: '&{XCLIENT ADDR=xxx LOGIN=dev NAME=[UNAVAILABLE] XCLIENT ADDR=xxx LOGIN=dev NAME=[UNAVAILABLE]}'
2023/02/22 09:17:08 [SMTP 172.18.0.1:48398] Sent 26 bytes: '500 Unrecognised command\r\n'
2023/02/22 09:17:08 [SMTP 172.18.0.1:48398] Connection closed by remote host
2023/02/22 09:17:08 [SMTP 172.18.0.1:48398] Session ended

I tried to find related information, but did not succeed in that. So I hope someone here could help me with that.