I'm looking for "best" way to populate treeview using threads.
I have multiple mail account which I'm checking for new emails.
My plan is to use
Your question is very broad, so this answer will also be.
tkinter doesn't play well with multi-threading. You can do it, but must make sure only the main thread interacts with the GUI. A common way to do this is to use the universal widget method
after() to schedule handling of data going out to or being retrieved from background threads, typically via
Queues, at regular intervals.