I understand that email from external was rejected to email group firstname.lastname@example.org.
It seems that the group posting permission configuration was incorrect.
Please kindly update the configuration to "public".
- Open Google Groups.
- Click My Groups > name of the group you want to edit.
- Click Manage in the upper-right corner.
- From the left-hand navigation menu, click Permissions > Basic Permission > Post > Public.
- Make your changes and click Save in the upper-left corner.