May I ask what features you consider essential are lacking in Thunderbird? I'm not a heavy e-mail user (for now, hopefully ever), and Thunderbird seems very feature-packed to me.

The only thing I'm afraid about is Thunderbird wiping one's inbox [^1] [^2] (both by the same user with discussion underneath)

[^1]: 2019 https://news.ycombinator.com/item?id=18807723#18808520

[^2]: 2025 https://news.ycombinator.com/item?id=44492682#44519053

Thunderbird does very poorly with both large inboxes (e.g., 20000+ emails) and large email messages (10+ MB). Search is also super slow.