Michael Fincham (michael) writes:
"The sending IP must have a PTR record (i.e., a reverse DNS of the
sending IP) and it should match the IP obtained via the forward DNS
resolution of the hostname specified in the PTR record. Otherwise, mail
will be marked as spam or possibly rejected."
They've been doing this for a while now, but must have only been
enabling it region by region at a time. Note this is only for v6.
Maybe there's some del^H^H^illusion it will help with spam (fat good
it did in v4).