SpamHaus expect the IPv6 /64 to be owned by the same entity.
This is not the case for Gandi VPS that are provided with a single
IPv6.
Gandi is working on it, they want to provide /64 to organisations, but
it's not ready yet.
In the meantime we're blocked by spamhaus since a few days on both the
/64 used by git.afpy.org and the /64 used by discuss.afpy.org.
So as a trash fix I propose sending emails using IPv4.