It looks like the mailinglist server IPv6 address has been blacklisted:
https://check.spamhaus.org/listed/?searchterm=2a04:3545:1000:720:acc1:5bff:fe5e:459
The 'Show details' shows the entire 2a04:3545:1000:720::/64 network is
blacklisted. Whois tells this is a network used by UpCloud Cloud
Servers. So either the mailinglistserver or a server it shares its
network with is compromised. If UpCloud gives its users their own /64,
it would probably be the former. Otherwise it might make sense to ask
them for one, as sharing IPv6 networks between customers is a no-no in
the IPv6 world.
