Please follow these instructions to install Mailtrack Add-on for Android:
1) Open Mailtrack for Android page in your Android phone or your Chrome browser in your desktop computer.
2) Press the install button.
3) Select the email account you want to use to send tracked emails using your Gmail app.
4) Press the allow button to accept the permissions.
5) If the install has been successful you should receive a confirmation message saying that Mailtrack for Android Gmail has been installed.
6) Re-start the Gmail app on your phone. In order to do so, please, close it completely and open it again. If you don't know how close completely your Gmail app, please, shut down your phone and switch it on again.