It’s not just Protonmail.

Blacklists like these aggressively and unapologetically collect all privacy-focused email domains they find, including simple forwarding and tagging services. With more and more sites using these lists to reject or black-hole email addresses, it has become difficult to protect one’s self from spam and cross-site account tracking.

Dear web developers, please don’t use these lists. Well-intended or not, they are privacy and user-hostile.

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It is logical that large corporations that base their economy on surveillance advertising hate users who protect their privacy by using all kinds of dirty tricks to bypass or eliminate these protections… Luckily I have had no problems so far with the Proton, Tuta and Murena (NextCloud) emails that I use in the EU.