On Mon, Jan 08, 2024 at 04:02:48PM +0100, Michael Grimm via Postfix-users wrote:

> Sometimes outgoing mail is deferred due to "reputational issues" at
> the receiving side. These "reputational issues" mostly concerned my
> IP6 addresses, thus I removed IP6 mailing completely. But now, I do
> want to give it a try, again.
> 
> In the past, whenever a mail has been deferred, I manually modified
> inet_protocols to the protocol *not* involved, restarted postfix and
> ran 'postqueue -f'. After having the "reputational issue" solved, I
> returned to inet_protocols=all.

You shouldn't need anything nearly so complex.

I have in master.cf:

    smtp       unix  -       -       n       -       -       smtp
    smtpv4     unix  -       -       n       -       -       smtp
        -o inet_protocols=ipv4
    smtpv6     unix  -       -       n       -       -       smtp
        -o inet_protocols=ipv6

For destination domains found to have issues with IPv6 (or conversely
with IPv4) just specify one of the alternative transports.

    gmail.com   smtpv4:gmail.com
    example.com smtpv6:example.com

Of course, as noted by Wietse, when MX-hosts for a myriad (not possible
to explicitly list) domains need a specific transport, the DNS reply
filter comes in handy.  Between that, and explicit transports you should
be able to have IPv4+IPv6 as a default with appropriate work-arounds.

Or perhaps IPv4 as a default, with IPv6 or both for domains where those
work better.

-- 
    Viktor.
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