As pointed out in the docs, setting ipv6 as preference is considered unsafe. As an IPv6 enthusiast, I had set my Postfix to ipv6 myself and remember some issues with broken MX.
The docs in question:
- smtp_address_preference (default: any)
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_protocols setting enables both IPv4 and IPv6.
Postfix SMTP client address preference has evolved. With Postfix 2.8 the default is "ipv6"; earlier implementations are hard-coded to prefer IPv6 over IPv4.
Notes for mail delivery between sites that have both IPv4 and IPv6 connectivity:
The setting "smtp_address_preference = ipv6" is unsafe. It can fail to deliver mail when there is an outage that affects IPv6, while the destination is still reachable over IPv4.
The setting "smtp_address_preference = any" is safe. With this, mail will eventually be delivered even if there is an outage that affects IPv6 or IPv4, as long as it does not affect both.
This feature is available in Postfix 2.8 and later.
But that note is rather confusing. The setting is describes as the address family to try *first*. Not as the only address family to try at all. So something in the docs is wrong. It's either the note or the documented behavior.
-- Maarten
