You need to first grant permission to "Notification accept " so your app can receive any notifications from device apps.
You need to follow the steps below to enable the "Notification accept" permission:
Setting => Apps => Special access => Notification accept
You need to give your application permission in AndroidManifest.xml:
   <service android:name="com.secondclone.UINotificationService"
    android:label="@string/app_name_notification"
    android:permission="android.permission.BIND_NOTIFICATION_LISTENER_SERVICE">
    <intent-filter>
    <action android:name="android.service.notification.NotificationListenerService" 
    />
    </intent-filter>
    </service>
Then, you write down the conditions to only receive notifications for new email notifications
/* This is the class that helps you receive notifications when there are new emails */
public class UINotificationService extends NotificationListenerService {
    @Override
    public void onCreate()
    {
        super.onCreate(); 
    }
@Override
public void onNotificationPosted(StatusBarNotification sbn)
{
    // Get notification of new messages of the Gmail app com.google.android.gm
    if (sbn.getPackageName().equals("com.google.android.gm"))
    {
       /* What you need to handle when a new email is here */
       Bundle extras = sbn.getNotification().extras;
                if (!contentGmail.equals(extras.getCharSequence("android.bigText").toString()))
                {
                    contentGmail = Objects.requireNonNull(extras.getCharSequence("android.bigText")).toString();
                    // This is the recipient's Gmail name information.
                    String mreceiver = extras.getString("android.subText");
                    // This is the sender's name.
                    String mSender = extras.getString("android.title");
                    // This is the Email subject.
                    String mSubject = Objects.requireNonNull(extras.getCharSequence("android.text")).toString();
                    // This is the text of this new mail.
                    String mContent = Objects.requireNonNull(extras.getCharSequence("android.bigText")).toString();
                    //Notification.EXTRA_TEXT
                    time = sbn.getPostTime() / 1000;
                    Log.i("tsMail", "Sender = " + mSender + " Receiver= " + receiver + " Content Gmail= " + mContent );
                    }
                }
    }
}
@Override
public void onNotificationRemoved(StatusBarNotification sbn) {
    Log.i("Msg","Notification Removed");
    }
}