How to setup Nginx as smtp proxy to external smtp server? - email

Question

I am trying to setup nginx mail proxy Gmail smtp server. Nginx server runs on docker.
During tests, I use openssl s_client -connect mail.localhost.com:12465 -starttls smtp (ports are different from config for port availability reasons).
Openssl connects successfully and I manage to get EHLO and run AUTH LOGIN. Still, I receive error 502 5.5.1 Unrecognized command. u6sm12628547ejn.14 - gsmtp. Same message appears when less secure apps security is on in account.
Running command above directly to smpt.gmail.com works great, and error is displayed correctly if less secure apps security is not allowed.
Current nginx.conf is below.
worker_processes 1;
events {
worker_connections 1024;
}
http {
include mime.types;
default_type application/octet-stream;
sendfile on;
keepalive_timeout 65;
server {
listen 9101;
server_name localhost;
location /smtpauthssl {
if ($http_auth_user ~* "(?<=#)(\S+$)" ) {
set $domain $1;
}
set $smtp_port '587';
add_header Auth-Status "OK";
add_header Auth-Port $smtp_port;
add_header Auth-Server "108.177.119.108";
return 200;
}
location = / {
return 403;
}
}
}
mail {
server_name mail.mydomain.com;
auth_http localhost:9101/smtpauthssl;
proxy_pass_error_message on;
proxy_smtp_auth on;
xclient on;
ssl_certificate /etc/ssl/certs/certificate.pem;
ssl_certificate_key /etc/ssl/certs/key.pem;
ssl_protocols TLSv1 TLSv1.1 TLSv1.2;
ssl_ciphers HIGH:!aNULL:!MD5;
ssl_session_cache shared:SSL:10m;
ssl_session_timeout 10m;
server {
listen 587 ssl;
protocol smtp;
smtp_auth login plain cram-md5;
}
}
My assumption is that something with logon is not passed to Gmail SMTP, but not sure what.
One more thing, what does Auth-Server header should contain, FQDN or IP address?