Function fetch_raw 

async fn fetch_raw(
    session: &mut Imap,
    uids: &[u32],
    whole: bool,
) -> Result<HashMap<u32, Option<Vec<u8>>>, SyncError>

Messages of the folder open in session, by UID: whole, or their header block (BODY.PEEK: nothing changes on the server), in memory only; a UID answered without them, None; one the server does not have, absent. A FETCH refused whole is asked again one message at a time; one refused alone counts as answered without it.