My protonmail account is free and I rarely sent emails with it. I receive more emails from bitcoindev announcement list and another newsletter which isn't crypto related. Now I'm really a bit disappointed of protonmail, especially when their own spam blocker doesn't quite explain what went wrong.

I can only guess that's intentional to avoid real spammers from trying to bypass it, but I've only sent about 14 emails for this campaign so far and it appears the blocking from protonmail started way earlier and I confused it with spam blocking of the registrars. This sucks from protonmail.