I've been using Aerion (https://aerion.3df.io) for a few months now, it's another mail+calendar+contacts app for Linux that is largely vibecoded. I haven't tested Penguin thoroughly yet, but I think Aerion is more feature rich, and has served me well over the months.

Why am I not using Thunderbird? Aerion's UI is much better, and most importantly snappier, especially on my older laptop.