https://git-send-email.io/
https://git-am.io/
Там даже есть пометки относительно Proton Mail:
Protonmail does not support the open, industry-standard protocols
necessary for git send-email to work out-of-the-box.
Be advised that Protonmail is generally known to be a pretty bad
email host. They will munge up your outgoing emails and your patches
may fail to apply when received by the other end. Not to mention
their mistreatment of open source and false promises of security!
You should consider a different mail provider.