AuthActivity

import android.accounts.Account;
import android.accounts.AccountManager;
import android.accounts.AccountManagerCallback;
import android.accounts.AccountManagerFuture;
import android.content.Intent;
import android.os.AsyncTask;
import android.os.Bundle;
import android.support.v4.app.FragmentActivity;
import android.util.Log;

import com.stackoverflow.ranjith.androidprojdel.backend.Constant;

public class AuthActivity extends FragmentActivity {

    private static final int AUTHORIZATION_CODE = 1993;
    private static final int ACCOUNT_CODE = 1601;
    private AuthPreferences authPreferences;
    private AccountManager accountManager;

    private final String SCOPE = Constant.GMAIL_COMPOSE + " " + Constant.GMAIL_MODIFY + " " + Constant.MAIL_GOOGLE_COM;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        accountManager = AccountManager.get(this);


        authPreferences = new AuthPreferences(this);

        if (authPreferences.getUser() != null
                && authPreferences.getToken() != null) {
            doCoolAuthenticatedStuff();
        } else {
            chooseAccount();
        }

    }

    private void doCoolAuthenticatedStuff() {
        //new senmailAsync().execute();   uncomment to send mail
    }


    private void chooseAccount() {
        Intent intent = AccountManager.newChooseAccountIntent(null, null,
                new String[]{"com.google"}, false, null, null, null, null);
        startActivityForResult(intent, ACCOUNT_CODE);
    }

    private void requestToken() {
        Account userAccount = null;
        String user = authPreferences.getUser();
        for (Account account : accountManager.getAccountsByType("com.google")) {
            if (account.name.equals(user)) {
                userAccount = account;
                break;
            }
        }

        accountManager.getAuthToken(userAccount, "oauth2:" + SCOPE, null, this,
                new OnTokenAcquired(), null);
    }

    /**
     * call this method if your token expired, or you want to request a new
     * token for whatever reason. call requestToken() again afterwards in order
     * to get a new token.
     */
    private void invalidateToken() {
        AccountManager accountManager = AccountManager.get(this);
        accountManager.invalidateAuthToken("com.google", authPreferences.getToken());
        Log.v("ranjapp", "invalidating token............");

        authPreferences.setToken(null);
    }

    @Override
    protected void onActivityResult(int requestCode, int resultCode, Intent data) {
        super.onActivityResult(requestCode, resultCode, data);

        if (resultCode == RESULT_OK) {
            if (requestCode == AUTHORIZATION_CODE) {
                requestToken();
            } else if (requestCode == ACCOUNT_CODE) {
                String accountName = data
                        .getStringExtra(AccountManager.KEY_ACCOUNT_NAME);
                authPreferences.setUser(accountName);

                // invalidate old tokens which might be cached. we want a fresh
                // one, which is guaranteed to work
                invalidateToken();

                requestToken();
            }
        }
    }

    private class OnTokenAcquired implements AccountManagerCallback<Bundle> {

        @Override
        public void run(AccountManagerFuture<Bundle> result) {
            try {
                Bundle bundle = result.getResult();

                Intent launch = (Intent) bundle.get(AccountManager.KEY_INTENT);
                if (launch != null) {
                    startActivityForResult(launch, AUTHORIZATION_CODE);
                } else {
                    String token = bundle
                            .getString(AccountManager.KEY_AUTHTOKEN);
                    Log.v("ranjapp", "Getting new token............");
                    authPreferences.setToken(token);

                    doCoolAuthenticatedStuff();
                }
            } catch (Exception e) {
                throw new RuntimeException(e);
            }
        }
    }


    private class senmailAsync extends AsyncTask<Void, Void, Void> {
        @Override
        protected Void doInBackground(Void... params) {
            GMailSender gMailSender = new GMailSender();
            gMailSender.sendMail("hi", "hi", authPreferences.getUser(), authPreferences.getToken(), "[email protected]");
            Log.v("ranjapp", "sent mail " + authPreferences.getUser() + "  " + authPreferences.getToken());
            return null;
        }
    }
}

AuthPreferences

import android.content.Context;
import android.content.SharedPreferences;
import android.content.SharedPreferences.Editor;
import android.util.Log;

public class AuthPreferences {

    private static final String KEY_USER = "userid";
    private static final String KEY_TOKEN = "mytoken";

    private SharedPreferences preferences;

    public AuthPreferences(Context context) {
        preferences = context.getSharedPreferences("authoris", Context.MODE_PRIVATE);
    }

    public void setUser(String user) {
        Editor editor = preferences.edit();
        editor.putString(KEY_USER, user);
        Log.v("ranjapp", "User is " + user);
        editor.apply();
    }

    public void setToken(String password) {
        Editor editor = preferences.edit();
        editor.putString(KEY_TOKEN, password);
        Log.v("ranjapp", "Password is " + password);
        editor.apply();
    }

    public String getUser() {
        return preferences.getString(KEY_USER, null);
    }

    public String getToken() {
        return preferences.getString(KEY_TOKEN, null);
    }
}

Constant

public static final String MAIL_GOOGLE_COM = "https://mail.google.com";
public static final String GMAIL_COMPOSE = "https://www.googleapis.com/auth/gmail.compose";
public static final String GMAIL_MODIFY = "https://www.googleapis.com/auth/gmail.modify";

GMailSender

import android.util.Log;

import com.sun.mail.smtp.SMTPTransport;
import com.sun.mail.util.BASE64EncoderStream;

import java.util.Properties;

import javax.activation.DataHandler;
import javax.mail.Message;
import javax.mail.Session;
import javax.mail.URLName;
import javax.mail.internet.InternetAddress;
import javax.mail.internet.MimeMessage;
import javax.mail.util.ByteArrayDataSource;

public class GMailSender {
    private Session session;


    public SMTPTransport connectToSmtp(String host, int port, String userEmail,
                                       String oauthToken, boolean debug) throws Exception {
        Log.v("ranjapp", "came to connecttosmtp");

        Properties props = new Properties();
        props.put("mail.smtp.starttls.enable", "true");
        props.put("mail.smtp.starttls.required", "true");
        props.put("mail.smtp.sasl.enable", "false");
        //props.put("mail.imaps.sasl.mechanisms.oauth2.oauthToken", oauthToken);
        session = Session.getInstance(props);
        session.setDebug(debug);

        final URLName unusedUrlName = null;
        SMTPTransport transport = new SMTPTransport(session, unusedUrlName);
        // If the password is non-null, SMTP tries to do AUTH LOGIN.
        final String emptyPassword = null;
        transport.connect(host, port, userEmail, emptyPassword);
        Log.v("ranjapp", "came before gen response");
        byte[] response = String.format("user=%s\1auth=Bearer %s\1\1", userEmail, oauthToken).getBytes();
        response = BASE64EncoderStream.encode(response);

        Log.v("ranjapp", "came to call issuecommand " + transport.isConnected());
        Log.v("ranjapp", new String(response));

        transport.issueCommand("AUTH XOAUTH2 " + new String(response), 235);

        Log.v("ranjapp", "came after issue command");
        return transport;
    }


    public synchronized void sendMail(String subject, String body, String user,
                                      String oauthToken, String recipients) {
        try {
            SMTPTransport smtpTransport = connectToSmtp("smtp.gmail.com", 587, user, oauthToken, true);

            MimeMessage message = new MimeMessage(session);
            DataHandler handler = new DataHandler(new ByteArrayDataSource(body.getBytes(), "text/plain"));
            message.setSender(new InternetAddress(user));
            message.setSubject(subject);
            message.setDataHandler(handler);

            message.setRecipient(Message.RecipientType.TO, new InternetAddress(recipients));

            smtpTransport.sendMessage(message, message.getAllRecipients());

        } catch (Exception e) {
            Log.v("ranjith", e.toString());
        }

    }
}