Nginx as reverse proxy mail server host

[siva.pannier]

Thank you for your suggestions!

My understanding from your suggestions is that you do not want me to make
any corrections on the client code. I just need to make corrections on the
Nginx configuration as per the blog link. 

I am trying to understand that blog, going through again and again. so far I
understand that it creates a SSL layer first through which it accepts the
client request. Client should point to my proxy host and one of the ports
listed under "mail{... }". Proxy server identifies the upstream host based
on the username came from the client request. Then the call is routed to
actual upstream host based on the port. Please correct me if I am wrong
anywhere.

My questions are 
1) Significance of this line "auth_http  
localhost:9000/cgi-bin/nginxauth.cgi;" is just to have my own authorization
logic and return the valid upstream server host based on the username. Is it
correct?
2) I want to know what does this mean "smtp_auth  login plain cram-md5;".
Does the connection to actual upstream happen here?

Please help me on this and also share links supporting the above
configuration.

Posted at Nginx Forum: https://forum.nginx.org/read.php?2,288532,288539#msg-288539