Somewhat unsurprisingly the usual Google Cars/Buses/Streetsigns recaptcha is used by Proton Mail once you go past a certain amount of signups per IP. There is

Somewhat unsurprisingly the usual Google Cars/Buses/Streetsigns recaptcha is used by Proton Mail once you go past a certain amount of signups per IP. There is simply no escaping this reCaptcha. Once in a while you hit some bad ones and have to waste 5 minutes or just give up on a particular service.