I am back from my vacation and working through all the mail I got in the two weeks. Amongst others I found a funny email bounce for the forum’s confirmation message: “We are not aware of anyone in Germany needing to email us”. That is already weird enough but ok — the forum sends mails using a DE domain as a sender, maybe they don’t like that. So I forwarded the mail using my @adblockplus.org address. The new bounce message was: “We are not aware of anyone in Norway needing to email us”. This time I finally got it — Americans only want to speak to other Americans :)
"Don't mail us unless you are in the US"