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Proton Mail Discloses User Data Leading to Arrest in Spain
(restoreprivacy.com)
This is a most excellent place for technology news and articles.
But if you use their service for free, you do not have to provide any identifying info. As far as I am aware there is no check what you enter is legit and there is no requirement to supply a backup address. So the whole solution for a user to stay anonymous as much as they can with Protonmail is simply to not enter any identifying info.