Apparently one needs to configure `-o smtpd_upstream_proxy_protocol=haproxy`
for other inet services as well in order to actually make it through.
This patch enables it on smtp and submissions ports.

The hint for that came from experimenting and the docker-mailserver project.

References:
https://docker-mailserver.github.io/docker-mailserver/edge/config/advanced/kubernetes/#proxy-port-to-service-via-proxy-protocol