Nginx as incomming mail server relay ?

Mail server relay.
I was happy to learn that nginx had mail proxy facilities.....

i would like to use nginx (ipv4/ipv6) incomming mail smtp frontend (pointed to by mx records)
my email server is older and can only handle ipv4 in that facility it is actualy listing to ipv4 port 2525 for(relayed)) incomming mail.
(ipv4 port 25 blocked by isp) ipv6 port 25 NOT BLOCKED)
I would like to be able to catch all the direct ipv6 mail trafic and relay the incomming mails through to ipv4 windows server.
I'm at this stage not interested in pop nor imap services (only incomming smtp from the MX record)

i've setup nginx mail service but not getting anything through ; not even on telnet nginx 25.
http {
server {listen 80;
listen 8080;
listen [::]:80;
listen [::]:8080;
}
http seems to work including php is not the issue here.
}

mail { server_name mail.* ;#non HTTP but MAIL smtp server
auth_http localhost:80/soa/mailauth.php;# return the correct address of the mail server
# the php is never called
# using php fpm-php over port localhost:9000 in http

#smtp_auth login plain; ?? authorisation not needed for incomming mail
#smtp_capabilities "SIZE 104857600" "VRFY" "ETRN "ENHANCEDSTATUSCODES" "8BITMIME" "DSN";
#imap_capabilities "IMAP4rev1" "UIDPLUS"
server {
listen 25; #mainly internal use
listen 2525; #x cross meail relay
listen [::]:25; # direct port 25 ipv6
# listen [::]:2525;# not used compatibility
protocol smtp;
}
resolver ipv4email-localip ;#?
# resolver [ipv6email-localip];#?
timeout 5s;
proxy on;
xclient on;
smtp_auth none;
}

I'm not an expert in nginx,
Can i do this ??
goal: Relay ipv6 mail-> ipv6 mx record -> ipv6 nginx server ==> ipv6 nginx(linux) ========> ipv4 mail server (windows) ????
alternatives are welcome but the router(bginx) is a minimal system (single core 1 gig mem)

can anyone help to get the config right