-
Mailspring
:love_letter: A beautiful, fast and fully open source mail client for Mac, Windows and Linux.
-
InfluxDB
Power Real-Time Data Analytics at Scale. Get real-time insights from all types of time series data with InfluxDB. Ingest, query, and analyze billions of data points in real-time with unbounded cardinality.
There's Mailspring, which is partially open-source/freemium. I've tried it and it's fast and looks nice, but doesn't support encryption as far as I know, export options are limited, and the weird proprietary features are pushed just a bit.
Mailpile.is comes to mind. Their Twitter account is inactive since 2018, but they did fix some things on Github in November of 2020: https://github.com/mailpile/Mailpile/
Developement of the email app is still pretty active as you can see by the commit log in it's github page. https://github.com/LineageOS/android_packages_apps_Email