其实只需要按照邮件发送的格式构建内容发送，对方邮件服务器能正常解析即可。

public static void send() {
    String sender = "sender@link-nemo.com";
    String receiver = "receiver@link-nemo.com";
    String password = "xxxxxxxxxx";
    String user = new BASE64Encoder().encode(sender.substring(0, sender.indexOf("@")).getBytes());
    String pass = new BASE64Encoder().encode(password.getBytes());
    try {
        Socket socket = new Socket("smtp.link-nemo.com", 25);
        InputStream inputStream = socket.getInputStream();
        OutputStream outputStream = socket.getOutputStream();
        BufferedReader reader = new BufferedReader(new InputStreamReader(inputStream));
        //
        PrintWriter writter = new PrintWriter(outputStream, true);
        System.out.println(reader.readLine());
        //Hello
        writter.println("HELO Nemo");
        System.out.println(reader.readLine());
        //AUTH LOGIN
        writter.println("auth login");
        System.out.println(reader.readLine());
        writter.println(user);
        System.out.println(reader.readLine());
        writter.println(pass);
        System.out.println(reader.readLine());
        //Above   Authentication successful

        //Set mail from   and   rcpt to
        writter.println("mail from:<" + sender + ">");
        System.out.println(reader.readLine());
        writter.println("rcpt to:<" + receiver + ">");
        System.out.println(reader.readLine());

        //Set data
        writter.println("data");
        System.out.println(reader.readLine());
        //title
        writter.println("subject:Hello Nemo,It's me!!!");
        writter.println("from:" + sender);
        writter.println("to:" + receiver);
        writter.println("Content-Type: text/plain;charset=\"utf-8\"");
        writter.println();
        //content
        writter.println("How do you do?");
        writter.println(".");
        writter.println("");
        System.out.println(reader.readLine());

        //Say GoodBye
        writter.println("rset");
        System.out.println(reader.readLine());
        writter.println("quit");
        System.out.println(reader.readLine());
    } catch (Exception e) {
        e.printStackTrace();
    }
}