> For security reasons, we can't say too much, but some truly massive residential IP botnets have appeared in recent years and can make millions of attempts per day

Ah yes. All those insecure IoT and unpatched/unpatchable routers that are discoverable on shodan and ultimately end up joining giant botnets. They are a plague not just to ProtonMail.

TBH, I’ve never seen a Captcha. But then I’d tend to use your service via mutt/bridge or the iOS app. And I have MFA enabled.