Both auto-forwarding and auto-reply are paid features, which makes cancelling & switching much more difficult. Gmail is a breeze comparatively. I highly recommend against using their addresses (e.g. protonmail.com, proton.me, pm.me)
Email forwarding is available for everyone with a paid Proton Mail plan.
(source)
I use it on Thunderbird. I just couldn’t grow to like thunderbird.
i couldn’t set it up and it went a little beyond my understanding why, but apparently it’s a paid feature. i don’t necessarily love thubderbird either, but it does its job for me
Thunderbird doesn’t have your private key to decrypt your Proton emails. The key lives in your browser and in theory there’s no way to securely provide that key to Thunderbird so it can do the decrypting. There’s a special application they built for business owners who want this functionality, but by nature it breaks Proton’s security because the email content is then stored in plaintext (or close enough) so it’s not “secure” in the same sense Proton webmail is.
You have to use the Proton Mail Bridge
That’s the “special application” I mentioned, but it seems to have been updated since I last looked at it so it now offers the same level of encryption as the webmail app.
I would prefer to see it freely available, but it doesn’t seem foundational to using the service in any scenario - free accounts have the webmail and mobile clients, which are arguably both more flexible (and maintainable) than the Bridge.