Cloudflare should have an emergency hotline for situations like this. Charge half the ransom to handle the traffic for the duration of the attack. Offer contract afterwards.
We don't proxy smtp. There are solutions to deal with that in a hybrid way, though.
Reminds me of when Uber had that surge pricing scandal during the Sydney hostage crisis.
Unfortunately the only safe play is to give away the service for free (for duration of the attack). Which could be a solid marketing strategy, cloudflare's price point is reasonable enough that many would stick with their service even after the attack was over.
For all the people getting nasty and arm chair quarter backing this on little to no information or trying to claim credit for things they did not do- understand that once you start working in venture funded startups pretty much everyone knows each other and many people have worked together before.
Additionally, I don't see ProtonMail as the kind of company that'll let other third parties terminate their SSL connections/proxy all their traffic.