[Joeri]

I think an effective to-do list management implementation is tightly coupled with calendars...

That is a fascinating take, because I am of the opposite opinion. I don’t think a todo system that involves scheduling tasks can work well, because it is impossible to figure out ahead of time whether too many or too few tasks are scheduled on a day, and because it is hard to reprioritize scheduled tasks. Instead, imho, the only role dates play in an effective todo system is to label tasks with their due date, but never to schedule a task on its due date (as that is too late) but only to inform the setting of priorities. Instead in my view the critical aspect of an effective todo system is a clear priority of tasks, easily reprioritized, so that working time involves only tackling the flat list of tasks in order of priority until the completion of the working day.

For me integration with email instead of calendar is super convenient though, because that is where tasks often stream into my todo list. But I never use unread or flagged status to mark mails as a todo, because email does not afford reprioritizing. I see email as a way station between other people and my todo list.