Below, please see how to edit a google group setting so external users can email to groups
By default, groups are restricted so that only users within your school can send emails to them. This protects your users from unwelcome spam messages.
To edit a group so that external users can send emails ti it follow these instructions.
1 navigate to admin.google.com
2 Use the search box to search for a group. Click the group from the list
3 Edit the group settings by clicking Access Settings link
4 Tick the External > Publish Posts box then hit the Save button at the bottom of the page as screenshot
External users can now send messages to that group
Ref: https://realsmart.groovehq.com/help/allow-external-users-to-email-groups