Notejoy allows you to add additional email addresses to your Notejoy account for the purpose of allowing you to forward email to Notejoy from additional email addresses. When you add additional email addresses, however, you still cannot login with those additional emails.
To add additional emails, head over to Email Settings. There you can add the email address. You'll be required to confirm the email address before you can start forwarding emails to firstname.lastname@example.org.