Since Protonmail has been a recent topic of discussion I wanted to share this thread from Reddit where a user had their email suspended for joining a community marketplace (video game based) that they had no emails received/sent from - the signup was the only interaction per the user.

Common ideas of why the ban occurred include a list of email addresses being sent to Protonmail as part of an active investigation to more privacy invading thoughts - I'm not sure you'll find a (satisfactory) answer in these threads but thought it was worth the read/sharing; the ban has since been reviewed and lifted.

The Proton team provided the following info:

OP was informed about the reason of his account suspension when he appealed. For privacy reasons, we cannot legally comment publicly or share further details about this beyond what has already been stated.

To clarify:

We do not suspend accounts for no good reason. What constitutes a permissible use of our service is clearly outlined in our Terms & Conditions: https://protonmail.com/terms-and-conditions

In the case of false positives, or accounts that may have been mistakenly disabled, users can appeal: https://protonmail.com/abuse Appeals are always reviewed by a person, not a machine.

There are reasons why we are often not able to give advance notice of account suspension. First, some account suspension happens algorithmically. For instance, if accounts are being used to send spam, we need to act immediately to prevent damage to Proton's IP reputation. Second, in cases like ransomware, if we give warning before account suspension, ransomware actors will not suffer any losses and therefore will continue to conduct illegal activities on ProtonMail. Third, there are also cases where due to court order, we are legally forbidden from informing users in advance.

Our anti-abuse systems and team are not foolproof, and we do make mistakes (although not in this specific case). However, we need to strike a balance between protecting our users, the integrity of our service and compliance with Swiss laws, on one side, and false positives (which we understand can be a source of major inconvenience).

See https://www.teddit.net/r/privacy/comments/l1ep9i/protonmail_disabled_my_account_due_to_illegal/

And for comments on Protonmails response see: https://www.teddit.net/r/ProtonMail/comments/l0jayb/protonmail_deleted_my_acct_after_it_was_reported/