Required an one time payment for free accounts
Protonmail should change requirements to get a free email be required a one time payment to combat fraud and stop protonmail's domains being blacklisted by email providers, I can't afford the current pricing but for a low one time fee to have a free email from you I would gladly pay, please consider this so your service is less used for nefarious purposes.

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pnobulls commented
if they would do this might loose lots of People, and they might go other places.
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Mcgiwer commented
The idea is stupid. When the user would need to pay for the account then he/she would probably resign from it. There is already a paid plan, so another is not nessecary, plus paying for a free account may cause a big facial and probably legal issues