I have many times tried finding a modern replacement for Thunderbird, but it always ends without success.
There’s Nylas Mail, Hiri etc, but there has always been some needed feature (for me) missing.
Then I heard about Monterail, which basically is a functional theme based on a Thunderbird mock up. And just recently an Arc (theme) integration for Thunderbird has been released.
The two together makes Thunderbird quite pleasant to look at.
