You guys are making things complex and confused ...

There's no trick to get postscreen working with haproxy.

*) Our haproxy.cfg snippet for port 25 (postscreen):

listen smtp_25
    bind :25
    ...
    server c77 c77.iredmail.org:25 send-proxy check on-marked-down shutdown-sessions
    ...

*) on c77.iredmail.org, Postfix main.cf:

postscreen_upstream_proxy_protocol = haproxy

And master.cf:

smtp      inet  n       -       n       -       1       postscreen
smtpd     pass  -       -       n       -       -       smtpd

That's it. Nothing complex, just add line in main.cf like the HAProxy blog says.

And, if you run postscreen in other port number, that's not involved in whole mail flow at all. IT MUST BE PORT 25.