enviar los emailes con la cuenta de gmail

SendGmail.java

import com.sun.mail.smtp.SMTPTransport;
import com.sun.mail.util.BASE64EncoderStream;
import org.json.JSONObject;

import javax.mail.Message;
import javax.mail.PasswordAuthentication;
import javax.mail.Session;
import javax.mail.Transport;
import javax.mail.internet.InternetAddress;
import javax.mail.internet.MimeMessage;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.URL;
import java.net.URLEncoder;
import java.util.LinkedHashMap;
import java.util.Map;
import java.util.Properties;

public class PasswordBaseEmail {
    private static Properties props;

    static {
        props = System.getProperties();
        props.put("mail.transport.protocol", "smtp");
        props.put("mail.smtp.starttls.enable", "true");
        props.put("mail.smtp.host", "smtp.gmail.com");
        props.put("mail.smtp.port", "587");
    }

    public static void main(String[] args) throws Exception {
        final String receiver = "xxxxx@gmail.com";

        // insecure acess
        // activa la opcion de Aplicaciones menos seguras https://myaccount.google.com/lesssecureapps
        String username = "xxxxx@gmail.com";
        String password = "******";
        sendEmailWithPass(username, password, receiver, "insecure-access");

        // application-wide pass
        // añade un app password https://security.google.com/settings/security/apppasswords, la opcion de verificacion en 2-pasos debe está activada
        username = "xxxxx@gmail.com";
        password = "******";
        sendEmailWithPass(username, password, receiver, "application-wide pass");

        // OAuth2
        // obtiene credencial desde https://console.developers.google.com/apis/credentials Create credentials > OAuth client ID
        final String clientID = "123456789...";
        final String secret = "***...";
        /*
        paso-1  obtiene authenticated-code que usaremos en el paso 2

        GET /o/oauth2/v2/auth?scope=https://mail.google.com/&response_type=code&redirect_uri=urn:ietf:wg:oauth:2.0:oob&client_id=<client-id> HTTP/1.1
        Host: accounts.google.com
        Connection: close


        paso-2  obtiene refresh-token

        POST /oauth2/v4/token HTTP/1.1
        Host: www.googleapis.com
        Content-Type: application/x-www-form-urlencoded
        Content-Length: 253

        code=<authenticated-code>&client_id=<client-id>&client_secret=<secret>&redirect_uri=urn:ietf:wg:oauth:2.0:oob&grant_type=authorization_code
         */
        final String refreshToken = "***...";
        sendMailWithOAuth(username, fetchAccessToken(clientID, secret, refreshToken), receiver);
    }

    /*
     paso 3 - obtiene access-token
     POST /oauth2/v4/token HTTP/1.1
     Host: www.googleapis.com
     Content-Type: application/x-www-form-urlencoded
     Content-Length: 206

     client_id=<client-id>&client_secret=<secret>&refresh_token=<refresh-token>&grant_type=refresh_token
     */
    private static String fetchAccessToken(String clientID, String secret, String refreshToken) throws IOException {
        Map<String, Object> params = new LinkedHashMap<>();
        params.put("grant_type", "refresh_token");
        params.put("client_id", clientID);
        params.put("client_secret", secret);
        params.put("refresh_token", refreshToken);


        StringBuilder postData = new StringBuilder();
        for (Map.Entry<String, Object> param : params.entrySet()) {
            if (postData.length() != 0) {
                postData.append('&');
            }
            postData.append(URLEncoder.encode(param.getKey(), "UTF-8"));
            postData.append('=');
            postData.append(URLEncoder.encode(String.valueOf(param.getValue()), "UTF-8"));
        }
        byte[] postDataBytes = postData.toString().getBytes("UTF-8");

        URL url = new URL("https://accounts.google.com/o/oauth2/token");
        HttpURLConnection con = (HttpURLConnection) url.openConnection();
        con.setDoOutput(true);
        con.setUseCaches(false);
        con.setRequestMethod("POST");
        con.getOutputStream().write(postDataBytes);

        BufferedReader reader = new BufferedReader(new InputStreamReader(con.getInputStream()));
        StringBuffer buffer = new StringBuffer();
        for (String line = reader.readLine(); line != null; line = reader.readLine()) {
            buffer.append(line);
        }
        JSONObject json = new JSONObject(buffer.toString());
        return json.getString("access_token");
    }

    private static MimeMessage buildMessage(Session session, String username, String receiver, String content) throws Exception {
        MimeMessage message = new MimeMessage(session);
        message.setFrom(new InternetAddress(username));
        message.setRecipient(Message.RecipientType.TO, new InternetAddress(receiver));
        message.setSubject("Test");
        message.setContent("<h1>" + content + "</h1>", "text/html");
        return message;
    }

    private static void sendEmailWithPass(final String username, final String password, final String receiver, String content) throws Exception {
        final Properties clone = (Properties) props.clone();
        clone.put("mail.smtp.auth", "true");
        Session session = Session.getInstance(clone, new javax.mail.Authenticator() {
            protected PasswordAuthentication getPasswordAuthentication() {
                return new PasswordAuthentication(username, password);
            }
        });

        final MimeMessage message = buildMessage(session, username, receiver, content);
        Transport.send(message);
        System.out.println("Done " + content);
    }


    private static void sendMailWithOAuth(String username, String accessToken, String receiver) throws Exception {
        Session session = Session.getDefaultInstance(props);
        session.setDebug(true);

        final MimeMessage message = buildMessage(session, username, receiver, "OAuth");

        SMTPTransport transport = new SMTPTransport(session, null);
        transport.connect("smtp.gmail.com", username, null);
        transport.issueCommand("AUTH XOAUTH2 " + new String(BASE64EncoderStream
                .encode(String.format("user=%s\1auth=Bearer %s\1\1", username, accessToken).getBytes())), 235);
        transport.sendMessage(message, message.getAllRecipients());
    }
}