Can’t speak for others but for me Thunderbird’s UI feels distracting and almost grating in some ways. It’s technically functional but doesn’t feel right.
I know things like that don’t bother some folks and that’s fine, but others like myself can’t turn the visual parts of their brains off and so some software feels like nails on a chalkboard to use.
The search is also incredibly slow. I didn't notice just how slow and inaccurate it is until I built my mail client on notmuch.
Maybe if you could spend some time defining the "feels right" aspect, this would go away.
I often find myself in situations where someone wants to break the UI I like, because they're used to something else.
It seems that most of the time, "feels right" just means "it works like I'm used to in other software". So: GMail, Outlook, whatever. Which is sad.
I wonder how would you react to "TheBat!" or Pegasus Mail?
> It’s technically functional but doesn’t feel right.
That might be the most accurate description of Thunderbird that I've come across. It's hard to pinpoint exactly what's wrong with Thunderbird, despite me using it every day. Somehow it feels like the emails aren't the important bit of the UI, but I also couldn't tell you what I'm looking at instead.