www-gem

I don’t know which OS you want this tool to run on, if you want/need a remote access, or the level of details you need for your tasks, but - since nobody mentioned it so far - I’d recommend giving Taskwarrior a look. You can also complement it with Timewarrior.

I second this recommendation. I don’t use the Disroot app on my phone, but they also offer a bunch of other great apps beyond emails in case you are interested. You can check them on their website. They’re all FOSS and free to use. Most of them don’t even require an account.