Plausible deniability
It may happen that you are forced by somebody to reveal the password to your protonmail account. There are many situations where you cannot refuse to reveal the password (for example, agencies or due to extortion). It would be important to have 2 mailbox passwords - one that leads to a standard mailbox and one to a "hidden" mailbox.
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crealis
shared this idea
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Ciro Santilli
commented
This secondary password should then lead to a fake empty inbox showing a setting where emails are deleted after 2 days. This way, if the attacker sends a test email, it will still show up, but being empty is plausible. Of course, this means that any new emails received will be visible by the attacker, so you have to find a way to inform senders that the account has been compromised.