The business for the web hoster is to provide me with some MBs for a website, plus a database, and also to provide me with some mailboxes on my domain.

Many small busineses have their 'online' presence like that. If I have mail deliverability issues, it's not my problem any more, I can call Hetzner. And before I started using it I was sceptical because if Hetzner did not fix the reputibility of their IPs or the spam from their other customers, it would be a problem for me. But so far, it worked quite well!