The email address in question was an unequivocal white-power slogan (https://www.splcenter.org/hatewatch/2017/10/10/when-white-na...) on a poster that called for white-power violence and repeated the same slogan. ProtonMail was quite specific about all of those things together being the cause of the ban.

Unless you're a perfectly innocent citizen who happens to use [email protected] for completely non-Nazi-related reasons, I don't think you have anything to worry about.