Which version of Postfix supports IPv6?

Postfix version 2.2 supports IPv4 and IPv6 on the following platforms:

AIX 5.1 and later
Darwin 7.3 and later
FreeBSD 4 and later
Linux 2.4 and later
NetBSD 1.5 and later
OpenBSD 2 and later
Solaris 8 and later
Tru64Unix V5.1 and later

For Postfix version 2.1 you should download patch from http://www.ipnet6.org/postfix/ to enable IPv6 support in postfix. There is patch available only for IPv6 and IPv6+TLS together. You should apply the patch to your postfix source code.

What are unsupported featureas known limitations of IPv6 support (both version and patch)

The 'smtp_host_lookup' parameter is not effective with IPv6. This is because a different lookup mechanism is used that cannot easily disable the 'local' (i.e., non-DNS) lookups. Whether local files or the DNS are used first, is determined by your operating system, e.g. in /etc/nsswitch.conf or /etc/host.conf.
The order of IPv6/IPv4 outgoing connection attempts is not yet configurable. First IPv6 is tried before IPv4.
No IPv6 open relay checks. Currently there are no blacklists that cover the IPv6 address space.
On Tru64Unix, when using 'mynetwork_style = subnet' (or: class), all prefixlengths are set to 128 (host-only) because co

de cannot determine the prefixlengths automatically.

IPv6 does not have class A, B, C, etc. networks. With IPv6 networks, the setting <tt>'mynetworks_style = class'</tt> has the same effect as the setting 'mynetworks_style = subnet'

How to enable IPv6 on Postfix?

It is automatically enabled if you compile postfix the above platforms.

How to configure IPv6 on Postfix?

You can configure the following parameter:

mynetworks: IPv6 network ranges are written in the form [ipv6:addr:range]/plen.
smtp_bind_address6: Source address for outgoing SMTP connections. The address is specified as a bracketed hex IPv6 address.
lmtp_bind_address6: Source address for LMTP client connections.