ERROR: this - can not be applied to

error appears in part of code where I have "this"

private void sendEmail() {
    //Getting content for email
    String email = editTextEmail.getText().toString().trim();
    String subject = editTextSubject.getText().toString().trim();
    String message = editTextMessage.getText().toString().trim();

    //Creating SendMail object
    SendMail sm = new SendMail(this,email,subject,message);

    //Executing sendmail to send email
    sm.execute();
}

My class SendMail is this

public class SendMail extends AsyncTask<Void,Void,Void> {
    //Declaring Variables

    private Context context;
    private Session session;

    //Information to send email
    private String email;
    private String subject;
    private String message;

    //Progressdialog to show while sending email
    private ProgressDialog progressDialog;



    public Context getContext() {
        return context;
    }

    public void setContext(Context context) {
        this.context = context;
    }

    @Override
    public void onPreExecute() {
        super.onPreExecute();
        //Showing progress dialog while sending email
        progressDialog = ProgressDialog.show(context,"Sending message","Please wait...",false,false);
    }

    @Override
    public void onPostExecute(Void aVoid) {
        super.onPostExecute(aVoid);
        //Dismissing the progress dialog
        progressDialog.dismiss();
        //Showing a success message
        Toast.makeText(context,"Message Sent",Toast.LENGTH_LONG).show();
    }

    @Override
    protected Void doInBackground(Void... params) {
        //Creating properties
        Properties props = new Properties();

        //Configuring properties for gmail
        //If you are not using gmail you may need to change the values
        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");

        //Creating a new session
        session = Session.getDefaultInstance(props,
                new javax.mail.Authenticator() {
                    //Authenticating the password
                    protected PasswordAuthentication getPasswordAuthentication() {
                        return new PasswordAuthentication(Config.EMAIL, Config.PASSWORD);
                    }
                });

        try {
            //Creating MimeMessage object
            MimeMessage mm = new MimeMessage(session);

            //Setting sender address
            mm.setFrom(new InternetAddress(Config.EMAIL));
            //Adding receiver
            mm.addRecipient(Message.RecipientType.TO, new InternetAddress(email));
            //Adding subject
            mm.setSubject(subject);
            //Adding message
            mm.setText(message);

            //Sending email
            Transport.send(mm);

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