I'm happy to announce it's now solved after my personal request to their owner!
But be aware it might explain why other services don't support Spamgourmet (I've bolded up the most surprising part):
Your mail host does reverse DNS lookups only via ipv6. We disable ipv6 by default due to infrastrudture software that we use not yet supporting it. This is the first time we’ve seen a host not support ipv4 for such lookups. Happy it’s fixed!