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If I check the IPv6 address:

nslookup 2400:cb00:472:ac46:f9d3
** server can't find 3.d.9.f.6.4.c.a.0.0.0.0.0.0.0.0.4.2.0.1.2.7.4.0.0.0.b.c.0.0.4.2.ip6.arpa: NXDOMAIN

It doesn't resolve, as the error states.

You may want to turn IPv6 off and run only IPv4.
For the mailserver at least.

See what happens.

Seems to be late at your place... you try to resolve the IP of Spamhouse...

I have a complete different IPV6 which resolves perfectly...