You can use this code MailSender.java
 import java.io.File;
 import java.io.UnsupportedEncodingException;
 import java.util.Properties;
import javax.activation.DataHandler;
import javax.activation.DataSource;
import javax.activation.FileDataSource;
import javax.mail.BodyPart;
import javax.mail.Message;
import javax.mail.MessagingException;
import javax.mail.Multipart;
import javax.mail.PasswordAuthentication;
import javax.mail.Session;
import javax.mail.Transport;
import javax.mail.internet.InternetAddress;
import javax.mail.internet.MimeBodyPart;
import javax.mail.internet.MimeMessage;
import javax.mail.internet.MimeMultipart;
import android.os.Environment;
import android.text.Html;
import android.util.Log;
public class MailSender extends javax.mail.Authenticator {   
    private String mailhost = "smtp.gmail.com";   
    private String password;   
    private Session session;   
    String to="";
    String sub="";
    String msg="";
    String file1="";
    String file2="";
    String user="";
    String pass="";
    public MailSender(String to, String sub, String msg,String file1, String file2,String user, String pass) {
        this.to=to;
        this.sub=sub;
        this.msg=msg;
        this.file1=file1;
        this.file2=file2;
        this.user=user;
        this.pass=pass;
    // TODO Auto-generated constructor stub
}
    public MailSender(String user, String pass)
    {
        this.user=user;
        this.pass=pass;
        // TODO Auto-generated constructor stub
    }
    public synchronized void sendMail() {   
        File mediaStorageDir = Environment.getExternalStorageDirectory();
        Properties props = new Properties();
        props.put("mail.smtp.host", "smtp.gmail.com");
        props.put("mail.smtp.socketFactory.port", "465");
        props.put("mail.smtp.socketFactory.class","javax.net.ssl.SSLSocketFactory");
        props.put("mail.smtp.auth", "true");
        props.put("mail.smtp.port", "465");
        if(user.trim().length()!=0 || pass.trim().length()!=0)
        {
        Session s1 = Session.getInstance(props,new javax.mail.Authenticator() {
            protected PasswordAuthentication getPasswordAuthentication() {
            return new PasswordAuthentication(user,pass);//change accordingly
            }
            });
        try {
            MimeMessage message = new MimeMessage(s1);
            InternetAddress fromAddress=new InternetAddress(user,"set your from text");
            message.setFrom(fromAddress);//change accordingly
            message.addRecipient(Message.RecipientType.TO,new InternetAddress(to));
            message.setSubject(sub);
            // message.setContent("<h1>This is actual message</h1>","text/html" );
            BodyPart messageBodyPart1 = new MimeBodyPart();  
            messageBodyPart1.setContent(msg,"text/html");
             Multipart multipart = new MimeMultipart();  
            multipart.addBodyPart(messageBodyPart1);  
            MimeBodyPart messageBodyPart2 = null;
            MimeBodyPart messageBodyPart3= null;
            if(file1.trim().length()>0)
            {
                 messageBodyPart2 = new MimeBodyPart();  
                String filename =file1;//change accordingly  
                DataSource source = new FileDataSource(filename);  
                messageBodyPart2.setDataHandler(new DataHandler(source));  
                messageBodyPart2.setFileName(filename);  
                multipart.addBodyPart(messageBodyPart2);  
            }
            if(file2.trim().length()>0)
            {
                 messageBodyPart3 = new MimeBodyPart();  
                String filename =file2;//change accordingly  
                DataSource source = new FileDataSource(filename);  
                messageBodyPart3.setDataHandler(new DataHandler(source));  
                messageBodyPart3.setFileName(filename);  
                multipart.addBodyPart(messageBodyPart3);  
            }
            message.setContent(multipart );  
            Transport.send(message);
            //Log.i("Status", "Message Sent Successfully");
            } catch (MessagingException e) 
            {
                throw new RuntimeException(e);
            } catch (UnsupportedEncodingException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }
        else
        {
            //Log.i("mail", user+pass);
        }
    }   
    protected PasswordAuthentication getPasswordAuthentication() {   
        return new PasswordAuthentication(user, password);   
    }   
}  
In your onclick method call asyntask like
 RetrieveFeedTask eft=new RetrieveFeedTask(receiver_email,sub,header,msg,"");
                    eft.execute();
Add  RetrieveFeedTask1 as inner class in your MainActivity and replace values of username and password
    class RetrieveFeedTask extends AsyncTask<String, Void, Void> {
    private Exception exception;
    String to="";
    String sub="hii";
    String msg="";
    String file1="";
    String file2="";
    String header="";
    String password="";
   // String user="";
    SharedPreferences shared = PreferenceManager.getDefaultSharedPreferences(MainActivity.this);
String user="from@gmail.com";//your sender email id
    String pass="password";//sender password
    //  String reversepass = new StringBuffer(pass).reverse().toString();
    //String text = new String(pass, "UTF-8");
    public RetrieveFeedTask(String to, String sub, String header,String msg,String file1) {
        this.to=to;
        this.sub=sub;
        this.header=header;
        this.msg=msg;
        this.file1=file1;
        // TODO Auto-generated constructor stub
    }
    public RetrieveFeedTask(String to, String sub, String header,String msg, String file1, String file2) {
        this.to=to;
        this.sub=sub;
        this.header=header;
        this.msg=msg;
        this.file1=file1;
        this.file2=file2;
        // TODO Auto-generated constructor stub
    }
    protected Void doInBackground(String... urls) {
        MailSender ms=new MailSender(to,sub,header+msg,file1,file2,user,pass);
        ms.sendMail();
        //ms.sendStastical(stastical_email, sub, header+msg);
        return null;
    }
    @Override
    protected void onPostExecute(Void result) {
     //   Log.d("postexecute", "p execute");
     //   pDialog.dismiss();
        Toast.makeText(getApplicationContext(), "Mail Sent Successfully", Toast.LENGTH_LONG ).show();
     File newfile=new File(file1);
     newfile.delete();
     File newfile1=new File(file2);
     newfile1.delete();
    }
     @Override
        protected void onPreExecute() {
            super.onPreExecute();
            Toast.makeText(getApplicationContext(), "Please wait while sending email", Toast.LENGTH_LONG).show();
        }
}