How to set up a mail server to use multiple domains and multiple ssl certificates?

Question

Nginx

Hello.
I set up a mail server based on this blog.xelfaer.ru/?p=492 article. And everything was fine and wonderful until the task came to raise a few more domains for mailing lists.
That is, the situation is such
3 domains:

• example.com - here is the usual mail correspondence
  
• subscribe .example.com - various mailing lists here
  
• tech .example.com - here are the mailing lists for tech. personnel
  

The server is currently configured for the example.com domain only. And in particular, an SSL certificate only for this name.
As it turned out, postfix out of the box does not know how to use a separate certificate for a separate domain. I would not want to buy a wildcard certificate just for the sake of making the mailer work on several domains.
I read on the Internet that people set nginx as a proxy (for example, here - blog.siphos.be/2012/12/nginx-as-reverse-smtp-proxy/), but something does not reach me how it works. Why does everyone turn off the imap_auth , smtp_auth options and turn on http_auth, with a link to some URL that inserts a couple of headers.
How is the link configured here between nginx and postfix .
There are several questions in total:

1. Is it even possible to implement the above through nginx-proxy?
   
2. Is there another way to implement our plans without raising another server and allocating another IP address