On 20 November 2021 20.15.17 UTC, Arjen de Korte <build+dovecot@de-korte.org> wrote:

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.