02-10 17:18:40.094 8690-8819/? E/AndroidRuntime: FATAL EXCEPTION: AsyncTask #1
     Process: com.example.appdeveloper.appname, PID: 8690
     java.lang.RuntimeException: An error occured while executing doInBackground()
         at android.os.AsyncTask$3.done(AsyncTask.java:300)
         at java.util.concurrent.FutureTask.finishCompletion(FutureTask.java:355)
         at java.util.concurrent.FutureTask.setException(FutureTask.java:222)
         at java.util.concurrent.FutureTask.run(FutureTask.java:242)
         at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:231)
         at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1112)
         at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:587)
         at java.lang.Thread.run(Thread.java:841)
      Caused by: java.lang.NoClassDefFoundError: com.example.appdeveloper.appname.EmailAutherticator
         at com.example.appdeveloper.appname.EmailHandler.sendMail(EmailHandler.java:55)
         at com.example.appdeveloper.appname.EmailHandler.doInBackground(EmailHandler.java:48)
         at com.example.appdeveloper.appname.EmailHandler.doInBackground(EmailHandler.java:25)
         at android.os.AsyncTask$2.call(AsyncTask.java:288)
         at java.util.concurrent.FutureTask.run(FutureTask.java:237)
         at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:231)
         at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1112)
         at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:587)
         at java.lang.Thread.run(Thread.java:841)

package com.example.appdeveloper.appname;

import android.content.Context;
import android.os.AsyncTask;
import android.widget.Toast;

import java.util.Objects;
import java.util.Properties;
import javax.mail.Address;
import javax.mail.Authenticator;
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;


public class EmailHandler extends AsyncTask<Objects, Void, Object> {

    private static String to = "receiver@example.com";
    private static String from = "sender@example.com";
    private static String subject = "Subject of email";
    private static String sender = "Sender of email";
    private static String mail = "Body of email";
    private static String username = "username";
    private static String password = "password";
    private Context context;


    EmailHandler(Context context) {
        this.context = context;
    }


    @Override
    protected Object doInBackground(Objects... objects) {
        sendMail();
        return null;
    }


    public void sendMail() {
        try {
            Authenticator auth = new EmailAutherticator();
            Properties properties = new Properties();
            properties.setProperty("mail.smtp.auth", "true");
            properties.setProperty("mail.smtp.starttls.enable", "true");
            properties.setProperty("mail.smtp.host", "smtp.gmail.com");
            properties.setProperty("mail.smtp.port", "587");
            properties.setProperty("mail.smtp.user", "username");
            properties.setProperty("mail.smtp.password", "password");
            Session session = Session.getDefaultInstance(properties,auth);

            MimeMessage message = new MimeMessage(session);
            message.setSubject("subject");
            message.setText("mail");
            Address address = new InternetAddress("from","sender");
            message.setFrom(address);

            InternetAddress ad[] = new InternetAddress[2];
            ad[0] = new InternetAddress("to");
            ad[1] = new InternetAddress("from");
            message.addRecipients(Message.RecipientType.TO, ad);
            Transport.send(message);
            Toast.makeText(context, "Sent!", Toast.LENGTH_LONG).show();
        }
        catch(Exception exp) {
            exp.printStackTrace();
        }
    }

}



class EmailAutherticator extends Authenticator {

    private String username = "username";
    private String password = "password";

    public EmailAutherticator() {
        super();
    }

    public EmailAutherticator(String user,String pwd){
        super();
        username = user;
        password = pwd;
    }

    public PasswordAuthentication getPasswordAuthentication(){
        return new PasswordAuthentication(username,password);
    }

}

apply plugin: 'com.android.application'

android {
    compileSdkVersion 23
    buildToolsVersion "25.0.2"

    defaultConfig {
        applicationId "com.example.appdeveloper.appname"
        minSdkVersion 19
        targetSdkVersion 23
        versionCode 1
        versionName "1.0"
        multiDexEnabled = true
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
}

dependencies {
    compile fileTree(include: ['*.jar'], dir: 'libs')
    testCompile 'junit:junit:4.12'
    compile 'com.android.support:appcompat-v7:23.4.0'
    compile files('libs/additional.jar')
    compile files('libs/mail.jar')
    compile files('libs/activation.jar')
}

dependencies {
    compile 'com.google.android.gms:play-services:9.0.0'
}