Postfix supports IPv6 from version 2.2 and up. By default only IPv4 support in enabled. Enabling IPv6 is as easy as adding one parameter to Postfix's main.cf:

/etc/postfix/main.cf:
    # You must stop/start Postfix after changing this parameter.
    inet_protocols = ipv4       (DEFAULT: enable IPv4 only)
    inet_protocols = all        (enable IPv4, and IPv6 if supported)
    inet_protocols = ipv4, ipv6 (enable both IPv4 and IPv6)
    inet_protocols = ipv6       (enable IPv6 only)

Be sure to also update match lists as mynetworks and debug_peer_list with relevant IPv6 addressing. http://www.postfix.org/IPV6_README.html