As separate apps, Apple's Calendar and Reminders have no problems with this, but it works fine in Fantastical as well, since they're handled separately there also. You won't likely be able to mix calendar events and tasks in the same calendar, but since Fastmail indexes event calendars and tasks calendars separately, you can give them the same name without any problems - although again, I'm not sure how this would be handled in Thunderbird. These lists won't show up anywhere in the Fastmail web interface, but they should sync just fine between all of your devices via CalDAV.

I haven't tried this in Thunderbird, but I assume the same would apply - just create a new CalDAV task list in Thunderbird, and it should sync up to Fastmail just fine. In my case, I was able to get both Fantastical and Apple Reminders to play nice with CalDAV tasks in Fastmail by making sure that I create the 'calendar' in those apps, since it needs to be flagged as a 'tasks' list as opposed to a calendar. That said, since Fastmail's CalDAV implementation adheres to proper RFC standards, it does allow you to create 'calendars' that contain tasks - you just can't do it from the Fastmail web interface. To create a task follow one of these steps: Click the File menu at the top of the Thunderbird window, or click the menu button. CalDAV calendars have a 'component type' flag that determines what data is stored in them, and since Fastmail doesn't have a task manager, the calendars that are created in the Fastmail web interface are designed to handle events and not tasks.

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