Function move_across 

pub fn move_across(
    from: &Account,
    from_password: &str,
    file: &Path,
    to: &Account,
    to_password: &str,
    folder: &str,
) -> Result<(), SyncError>

Moves the message stored at file into folder of another account: a copy is appended there, its flags kept, and only once that server has it is the original taken off the first one. Nothing is lost on the way: if the copy fails, the original stays; if removing the original fails, both remain.