Ask HN: How secure is Protonmail really?

... and should I move my custom domain over to it for email?

ProtonMail appears to be secure so far but a couple of weeks ago there were some accusations levelled against ProtonVPN by PIA. While a lot of it was debunked but anyone pursuing the thread would be left with a bit of cynicism.

But here’s the caveat, no matter how transparent they are or how many white papers they publish, there’s no way to verify what is actually behind the hosted service. If your threat model includes a nation state then anything short of self-hosting would be risky. If your threat model involves getting away from google and trackers then it should be safe bet. For that matter even other providers like Fastmail or Zoho would be a good choice.

"... and should I move my custom domain over to it for email?"

What are you trying to achieve? What are your goals?

That wasn't the question

Not sure what you mean by 'secure'. If you send a person an email with ProtonMail, it defeats all the security that ProtonMail has (unless it's to another PM email). You should only rely on yourself for security. Start using PGP and demand that all your friends/relatives use PGP. Only sign in through Tor, use different emails for different services, etc.

You can't rely on a company to provide you security or privacy.