1. Related code:
public static void sendSSLMessage (String hostName, int port, String username, String password, String subject, String massage, String. To) throws EmailException {

    Email email = new SimpleEmail();
    email.setCharset("UTF-8");
    email.setHostName(hostName);
    email.setSmtpPort(port);
    email.setSSLOnConnect(true);
    email.setAuthenticator(new DefaultAuthenticator(username, password));
    email.setFrom(username);
    email.addTo(to);
    email.setSubject(subject);
    email.setMsg(massage);
    email.send();
}

2. Screenshot:
3. There are multiple recipients, only one, 163 mailbox can show multiple, Ali enterprise mailbox only shows one