Function append_then_remove 

fn append_then_remove(
    from: &Account,
    from_password: &str,
    source: &Folder,
    origin: ImapOrigin,
    raw: &[u8],
    letters: &str,
    to: &Account,
    to_password: &str,
    folder: &str,
) -> Result<(), SyncError>

Copies raw into folder of to, its flags kept (Maildir letters), and only once that server has it takes the original off source of from.