Function fetched 

pub(crate) fn fetched(
    folder: &Folder,
    all: &[Folder],
    account: &Account,
) -> bool

Whether a folder is fetched: everything but views of other folders, except where “All Mail” is the only archive there is (Gmail): it is fetched then, and shown as the archive, without what the inbox and Sent already show. Folders you chose not to keep stay on the server; the inbox is always kept. A folder whose name would make it the Maildir itself or the folder above (“INBOX.” or “INBOX..” with “.” between levels) is not fetched.