If a user misses multiple payments, the amount due should be reduced to reflect the period that a user could not access their email.
When multiple payments have been missed, users are required to pay all past invoices in full to reinstate their account and view their email. While users should be expected to pay for storage costs during that period, they should not be expected to pay the invoices in full, as they did not receive 100% of the service during that period.

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Aleksandr Korneev commented
Totally agree with Wave. You should NOT be billed for storage and functionality you didnt use when proton effectively locks you out of your data for not paying your invoice. If you have skipped few months, only the latest invoice should be necessary to pay.
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R commented
This is a ridiculous idea. No company I know of works in this way. What's to stop you from just not paying for extended periods of time (barring "pay[ing] for storage costs"), and then just starting to pay again when you feel like using it more?
As counterexample, imaging you have a 24-month contract for a phone, and a few months into the contract you just decide to stop paying because you're not using the phone for a little while. Then you can surely expect to just start paying once more when you feel like using the (contracted) phone again, right? After all, if the phone company turns off your phone's access to the network until you pay (and because you haven't paid), surely you don't have to pay for the period of time that you couldn't use your phone?
Of course not. You've entered into a contract to pay for these services, and likewise with ProtonMail you've said you'll be paying for use of their services. You don't just get to say "but since I didn't pay, I didn't have access, therefore I have no obligations".