Protonmail seems to routinely get flagged by fraud, spam, and security systems. You emails are probably getting eaten by a security appliance before it even reaches the government exchange server or whatever.

Protonmail is one of the few remaining email hosts that let you create a free account without a verified mobile phone number. It probably does get used for spam quite a bit - relatively to gmail/outlook/etc., not in absolute terms that is, IME most spam is from compromised hosts and botnets. Then again, the major mail providers, especial Microsoft, are really trigger happy and don't seem to care about the damage they do to smaller providers so it could just as well be lazyness or intentional anti-competitive behavior. I think at some point we will need legislation to keep email an open federated system.

> It probably does get used for spam quite a bit - relatively to gmail/outlook/etc., not in absolute terms that is

Not really, big players have the advantage that nobody is going to block them. Most of them spam regularly by thousands of letters before some automation of theirs triggers (or the spammer stops).

That is not the case as this is messages sent TO protonmail. It may be related anti-whatever measures, but is not typical anti-spam.