Just a quick tip: You have Postfix running and want to enable IPv6 for your email as well? After quite some playing around I finally got it right:
inet_interfaces = 127.0.0.1, 184.108.40.206, 2a01:348:6:315::2, ::1 inet_protocols = ipv4, ipv6
This makes Postfix listen only where you actually need it to - I dont like daemons listening on interfaces where they are not serving any requests.
How do I test IPv6 enabled mail?
Just go to here and off you go. Dont forget to put a fancy text in there what you can show off with... *g