I like Thunderbird, it’s a great tool for private use. One killer feature I always missed (not sure if it exists today by default in Thunderbird), is the great calendar integration of outlook. I use the calendar a lot, during work but also to organize our family. It’s super important for me to able to send invites to co workers and my wife :-)

I've never seen a piece of software that managed to implement all of the iCalendar specification, which to me seemed like a data model for a good productivity app that's just never manifested. iCalendar (RFC 2445 from 1998) outlined not only events but todo and a journal component for memorializing meetings. Outlook seems to ignore VTODO entries in iCalendar completely, and VJOURNAL support is deprecated.

Since last year, TB has native caldav/carddav support.