activity_main.xmlMainActivity.java
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:padding="16dp"
    tools:context="com.codinginflow.emailintentexample.MainActivity">

    <TextView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="To:"
        android:textAppearance="@style/TextAppearance.AppCompat.Large" />

    <EditText
        android:id="@+id/edit_text_to"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:inputType="textEmailAddress" />

    <TextView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="Subject:"
        android:textAppearance="@style/TextAppearance.AppCompat.Large" />

    <EditText
        android:id="@+id/edit_text_subject"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:inputType="textEmailSubject" />

    <TextView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="Message:"
        android:textAppearance="@style/TextAppearance.AppCompat.Large" />

    <EditText
        android:id="@+id/edit_text_message"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:gravity="start|top"
        android:lines="10" />

    <Button
        android:id="@+id/button_send"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="send" />

</LinearLayout>

package com.codinginflow.emailintentexample;

import android.content.Intent;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;

public class MainActivity extends AppCompatActivity {
    private EditText mEditTextTo;
    private EditText mEditTextSubject;
    private EditText mEditTextMessage;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        mEditTextTo = findViewById(R.id.edit_text_to);
        mEditTextSubject = findViewById(R.id.edit_text_subject);
        mEditTextMessage = findViewById(R.id.edit_text_message);

        Button buttonSend = findViewById(R.id.button_send);
        buttonSend.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                sendMail();
            }
        });
    }

    private void sendMail() {
        String recipientList = mEditTextTo.getText().toString();
        String[] recipients = recipientList.split(",");

        String subject = mEditTextSubject.getText().toString();
        String message = mEditTextMessage.getText().toString();

        Intent intent = new Intent(Intent.ACTION_SEND);
        intent.putExtra(Intent.EXTRA_EMAIL, recipients);
        intent.putExtra(Intent.EXTRA_SUBJECT, subject);
        intent.putExtra(Intent.EXTRA_TEXT, message);

        intent.setType("message/rfc822");
        startActivity(Intent.createChooser(intent, "Choose an email client"));
    }
}