Hello, I'm trying to unify backend server maps in a local DNS server:

Internet HTTP -> reverse proxy [queries DNS] -> backend HTTP servers
Internet SMTP -> reverse proxy [queries DNS] -> backend SMTP servers
Internet POP3 -> reverse proxy [queries DNS] -> backend POP3 servers
Internet IMAP -> reverse proxy [queries DNS] -> backend IMAP servers

Instead of make a database map for each proxy, or querying specialized scripts, I'm setting up a virtualhost system based on proxies that query local backend servers as if they were in internet: with DNS answers.
I want to know if nginx could integrate a way to do this, without requiring a dedicated database or web service:

Internet IMAP petition (guided with internet DNS) ->
Public firewall with a single IP ->
nginx (guided with LAN DNS) ->
each backend server.

EXAMPLE 1:
MUA asks for imap.example.com
Internet DNS maps MUA to 80.80.80.80 ->
Firewall to the LAN ->
1 nginx asks login to MUA user
2 MUA gives username friend@example.com
3 nginx is wide configured to replace user@* by imap.*
and asks for imap.example.com
4 A local DNS answers imap.example.com A 192.168.1.5
nginx -> backend IMAP server 192.168.1.5

Thanks.