@Smirftsch Yeah, the idea of Dekko or another email client as a core app makes sense to me.
Longer term, I thought at one point I heard talk about trying to fork Thunderbird, possibly via Snaps, which will be enabled by default in 24.04. But I'm sure that'll require some work to function well in UT.
Meanwhile, I hope some other input about email usage, and Dekko specifically, can help make UT more useable for you.