AsyncHttpRequest.java

public class AsyncHttpRequest extends AsyncTask<String, Void, String> {

    private Context context;

    private AsyncHttpRequest(Context context) {
        this.context = context;
    }

    /**
     * 現在日時をyyyy/MM/dd HH:mm:ss形式で取得する.
     */
    private static String getNowDate(){
        final SimpleDateFormat df = new SimpleDateFormat("yyyy/MM/dd HH:mm:ss");
        final Date date = new Date(System.currentTimeMillis());
        return df.format(date);
    }

    @SuppressLint("UnlocalizedSms") @Override
    protected String doInBackground(String... params) {
        try {
            // ----- メール送信
            MailSender mailSender = new MailSender();
            mailSender.send(
                    params[0],
                    "Hello, " + params[1]  + "/r/n" +
                    "From " + params[2] + "/r/n" +
                    "Now Time: " + getNowDate()
            );
        } catch (Exception e) {
            return e.toString();
        }
        return null;
    }

    @Override
    protected void onPostExecute(String result) {
        Toast.makeText(context, "メールを送信しました", Toast.LENGTH_SHORT).show();
    }
}

MailSender.java

public class MailSender {
    private Properties properties;

    public MailSender(){
        properties = System.getProperties();
    }

    public void send(String title, String content){
        properties.put("mail.smtp.host", "smtp.gmail.com");
        properties.put("mail.smtp.port", "587");
        properties.put("mail.smtp.auth", "true");
        properties.put("mail.smtp.starttls.enable", "true");

        Session session = Session.getDefaultInstance(properties, new javax.mail.Authenticator(){
            protected PasswordAuthentication getPasswordAuthentication(){
                return new PasswordAuthentication("your_gmail_address@gmail.com", "your_account_password");		// ←修正
            }
        });
        MimeMessage message = new MimeMessage(session);

        try {
            String from = "xxxxxxxxx@xxxx.xxx";		// ←修正
            String[] to = {"yyyyyyyy@yyyy.yyy"};		// ←修正
            message.setFrom(new InternetAddress(from));
            InternetAddress[] toAddress = new InternetAddress[to.length];

            for(int i = 0; i < to.length; i++ ){
                toAddress[i] = new InternetAddress(to[i]);
            }

            for (InternetAddress toAddres : toAddress) {
                message.addRecipient(Message.RecipientType.TO, toAddres);
            }

            message.setSubject(title);
            message.setText(content);
            Transport.send(message);

        } catch (MessagingException e) {
            e.printStackTrace();
        }
    }
}