The 'From addresses' on your email replies to your customers will be shown as firstname.lastname@example.org by default. To update this address, you must finish the setup by verifying the email you setup forwarding from.
How can I verify the email forwarding?
Click on the inbox email from the left-side menu and you will see a yellow banner on top that asks you to finish the setup.
Click on the link which verifies the forwarding setup and will change the reply address of your email.