Edison Mail is one of them, but I would recommend against it due to a number of privacy scandals.
My personal recommendation would have to be Mozilla Thunderbird. It is completely free and open-source, not to mention that it has a focus on privacy (like Mozilla Firefox). If you end up liking it, you should really consider donating. Mozilla no longer funds it for a number of reasons, so the Thunderbird team relies on donations from the community to keep it going. There’s also a beta channel and a nightly channel of Thunderbird.
Here’s a list of notable features in Thunderbird (newsfeed, chat, calendar, task list, features built for macOS power users, etc).
In total, there are only 13 employees on the Thunderbird team, not to mention that there’s a tightly knit community.
