I use Thunderbird everywhere, but I want to contribute this to the conversation: you don’t have to have your email client open all day long. I open my email client few times a day, and that’s it. I do the same with my chats, but with the chats (especially the work ones) I’m expected to reply within minutes, unfortunately. And, well, that’s another topic.

For email, I mostly don’t care whether it takes too much RAM, if the app is usable. I work with it, then I close it. That’s my workflow, at least. I believe I’m not alone in this. My iPhone is the mini server that gets all the notifications for the emails I need. (By being connected with the default email client.) So, if I want to reply from my laptop, I’ll open my app. Otherwise it’s closed.