https://support.google.com/mail/answer/81126?hl=en#authentication see
"Additional guidelines for IPv6"
Right now, it's basically an OR between the two instead of AND, though as
with any rule, there are exceptions. We also still do "best guess" style
SPF auth, so if you set your PTR records right, you may be getting SPF for
"free" from us.
And it's always subject to change based on spamminess.
If your work around is to use ip4 to send to us, that's fine. The
percentage of spam mail on ip6 is higher than on ip4. Our own outgoing
code makes choices on whether to use ip6 or ip4 based on expected
authentication level, and it is ugly. I don't like ugly, but I don't like
spam either.
Brandon
On Mon, Mar 13, 2017 at 1:22 PM, Philip Homburg
<pch-ietf-6(_at_)u-1(_dot_)phicoh(_dot_)com>
wrote:
In your letter dated 13 Mar 2017 18:43:21 +0100 you wrote:
Nor is gmail, which also requires that incoming IPv6 mail be authenticated
with SPF or DKIM. They know what they are doing, and they have decided
that the amount of legit mail they will lose by doing this is
insignificant compared to the improvement in the amount of spam and
malware they will be able to filter.
That's not correct, unless gmail made an exception for my home mail server.
Getting mail delivered to gmail over IPv6 works most of the time without
ever setting up SPF or DKIM. Gmail does seem to be the single most
unreliable mail server that I know of, mostly due to their attempts
to be more strict on IPv6.
What gmail does require is reverse DNS for IPv6.