mouss:
> RFC 5321 (the same sentence is in 2821) says
> "In any case, the SMTP client SHOULD try at least two addresses."
>
> so a client that only tries two addresses is compliant. If you use
> twenty IPs, be prepared to see clients ignore most of them (and no
> tuning of _your_ postfix will help).
As of snapshot 20100306, Postfix's hard-coded "try ipv6 before
ipv4" behavior is now configurable. The default setting is
backwards-compatible with the behavior that was imported long ago
with the TLS+IPV6 patch.
I may change the default into "any" (i.e. toss a coin between IPv6
and IPv4) to improve the worst-case behavior. If there is a problem
that affects only one protocol, then mail will eventually get
through via the other protocol.
Of course, this has no effect on IPV4-only hosts.
Wietse
Quoting from the friendly manual:
smtp_address_preference (default: ipv6)
The address type ("ipv6", "ipv4" or "any") that the Postfix SMTP client
will try first, when a destination has IPv6 and IPv4 addresses with
equal MX preference. This feature has no effect unless the inet_proto-
cols setting enables both IPv4 and IPv6.
This feature is available in Postfix 2.8 and later.
