i use Zimbra every day in my company but the TODO management is not very well. It starts with the sorting. The TODO's should get sorted by time added and maturity date. So if no maturity date is existent the task should get sorted with the creation date.
But it is totally idiocy to show done tasks at the same place as undone tasks. Done tasks should be visible at the bottom of all others. Also it should be possible to assign an task to an calender item and tasks with maturity dates should be visible below the date in the calender like in outlook.
Also it should be possible to have tasks that depends on other tasks.