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 (

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.