[Cameron_D]

Cloudflare don't proxy mail though, which is ProtonMail's main business, so that wouldn't have done much for keeping their services up.

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.

[cbg0]

They have BGP origin protection, where they announce your IP space for enterprise plan, probably expensive though.