By default, sending to a group outside your domain is not allowed.
As an owner of the group or as a super admin in your domain, you can allow public sending to a group by following these instructions:
1. Log in to a group you want to administer (groups.google.com)
2. Go to settings: Gear-icon -> Group settings
3. On the left navigation, select Permissions -> Basic permissions -> Post -> Select group of users -> Select public.
4. Click Save on the top part of the screen.