Hi mallen,
From my understanding, you’d like to create an email address which can meet the following scenario:
When an email is sent to this specific email address, many users (some are in your organization while others don’t) can receive the email. Also, both internal users and external uses should be able to send to this email address. If I misunderstand it, please feel free to correct me.
If this is the scenario, please follow the steps below:
1. Log in to Office 365 portal (https://portal.microsoftonline.com/ ) with your admin account.
If you are an Office 365 E/M plan user, go to Admin -> Exchange to enter EAC (Exchange Admin Center).
If you are an Office 365 P plan user, click Outlook to OWA (Outlook Web App) and change the URL from https://outlook.office365.com/owa/.. to https://outlook.office365.com/ecp.
2. Navigate to recipients->contacts, click the add icon to add users whose email addresses are not at your domain as mail contacts.
3. Navigate to recipients->groups, click the add icon, select Distribution Group. Then add the newly created mail contacts and the internal users as members to this distribution group. Click Save.
Note: The owner of the DG will appear in the member list of the dg, he will receive the mails, too. If you don’t want this situation occur, please remove the owner from the member list or un-tick ‘Add group owners as members’ when creating the distribution group.
4. After the DG is created, double click the distribution group and navigate to delivery management. Check Senders inside and outside and outside my organization. Click Save.
Thanks,
Xinyu Zhou