@Kubernetes Alright, thanks! Going from the last update entry in my Notifications tab, I think I am on version 7.4.3 - not sure how to definitely check that. But that would explain the problem!
Found the version in the settings tab & will update!
Some more info: When I add my home IP to my SPF record, it does no longer SoftFail, but it still doesn't arrive in my ProtonMail inbox. I'll keep trying 