Function isolate 

async fn isolate(
    server: &Server,
    password: &str,
    account: &Account,
    own: &[String],
    gmail: bool,
    dir: &Path,
    folder: &Folder,
    validity: u32,
    uids: &[u32],
    update: &mut Update,
) -> Result<usize, Stop>

A broken batch’s messages, one at a time, each on a working connection: one that breaks it alone, twice, is set aside (skipped), so that it never holds back the rest of its folder. When none of them comes, the fault is the server’s or the line’s, not a message’s: nothing is set aside, and the error is said. The connections it made.