Here’s a non-GUI application that functions well: ProtonMail Bridge.
I got ProtonMail Bridge running on the Pinephone and was able to sync with Geary and Evolution. If someone who already has their phone could confirm it works for the L5, that would be appreciated. Bridge seems to be fully functional. It’s not officially supported for arm64 and you won’t find it packaged in any repositories, but cloning the source and building with
GOARCH=arm64 make build-nogui works without any problems once you have the build toolchain. The resulting binary can be run with
protonmail-bridge & to run as a background process or
protonmail-bridge -c for CLI configuration.