Wietse Venema via Postfix-users:
> Wietse Venema via Postfix-users:
> > It's a rather long explanation for "why not do X". like several
> > times longer than the text that explains what protocol preferences
> > do. And this is the only place where adding that text would help.
> 
> I updated the text a little:

I'm adding a section to smtp_address_preferences, about when not
to use this setting.

        Wietse

Notes for mail delivery between sites that have both IPv4 and IPv6 connectivity:

  * Some remote SMTP servers will flag messages received over IPv6
    as more 'spammy' than messages received over IPv4. This may be
    because the Postfix SMTP client's IPv6 address lacks a proper
    PTR record, or because the client shares an IPv6 address block
    with bad users.

    To achieve consistent delivery performance, don't tweak the
    smtp_address_preference setting; instead, receive mail over
    both IPv4 and IPv6, and deliver mail only over IPv4.

    /etc/postfix/main.cf:
        inet_protocols = all

    /etc/postfix/master.cf
        smtp ...other fields... smtp -o inet_protocols=ipv4

  * The setting "smtp_address_preference = ipv6" is unsafe. [Existing
    text omitted for brevity]
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