Nginx as mail proxy: different domains with different certs

[PGNet Dev]

> Name-based (including SNI-based) virtual servers are not supported
> in the mail proxy module.  As such, the remaining options are:
> 
> - Use multiple names in a certificate
> - Use IP-based (or port-based) virtual servers
> 
> You can combine both options as appropriate.

add'l useful option for mail proxy + sni

   https://www.linuxbabe.com/mail-server/smtp-imap-proxy-with-haproxy-debian-ubuntu-centos