I switched from ProtonMail after a few months (despite having paid up front for a year) because the UX is so bad. You are forced to use their client apps, which are full of annoying bugs and design flaws. They ignored my support requests. Eventually I switched to MailFence, which has even worse UIs, straight out of the 90s, but you can use any IMAP client so it doesn’t really matter. (ProtonMail does have a bridge tool so you can use IMAP, but only on a desktop OS.) Still on the lookout for something better.

For desktop ProtonMail has a bridge to allow you to use standard imap and smtp: https://protonmail.com/bridge/

I guess for mobile phones you need to use their app.

Protonmail has also given in and paid out thousands* to criminals who DDoS their systems, no?

*In my original post I mistakenly wrote "millions"

(Note: The parent comment claimed that ProtonMail paid "millions" in ransom. It appears to have been edited to say "thousands" now.)

I am not affiliated with ProtonMail, but the answer is no. ProtonMail has paid a ransom exactly once, back in 2015 when it was just launched and suffered a DDOS attack that caused a lot of collateral damage to third parties, who pressured ProtonMail to pay the ransom.

ProtonMail has since committed to "never pay another ransom" and invested in advanced DDOS mitigation capabilities, including by becoming its own ISP.

Sources:

https://protonmail.com/blog/protonmail-ddos-attacks/

https://protonmail.com/support/knowledge-base/email-ddos-pro...

All I’ve heard is they once paid $6000 to stop a ddos in 2015 at the demand of their hosting isp.