Software Freedom Conservancy has a pretty nice page called Give Up GitHub!, definitely check that out
My company uses Roundcube for Webmail and offers Thunderbird as a native client. It’s always great to see free software in a corporate environment.
Roundcube should work pretty well for you: https://roundcube.net/
They probably wanted to save money on support staff, now they will get a massive OpenAI bill instead lol. I find this hilarious.
If you use a Google Pixel Tablet, you can install GrapheneOS and revoke Sensor permissions for all apps.
SAP, closely followed by IBM/Lotus/(I have no idea which random company they were sold to) Notes
I fucking hate this corporate bullshit software