Email message sent from outside of OA to OA-managed mail-enabled Hosted Exchange public folder or distribution list is not delivered. Instead, a sender receives an NDR (Non-delivery report) with the
MapiExceptionNotAuthorized error message in it.
Security setting configured in Exchange prevent the recipient from receiving email messages to their public folder or distribution list.
In OA Customer Control Panel open the distribution list in question and check the Security tab. Verify that Accept messages from is set either to Everyone, or to Members and the external user which cannot send email to distribution list is added to members list:
If this does not help, it is possible that restrictions were applied outside of POA. You can check Message Delivery Restrictions in the Mail Flow Settings tab for the distribution list if you prefer to use EMC (Exchange Management Console), or use the Get-DistributionGroup cmdlet and filter for "*AcceptMessages*" fields in in the EMS (Exchange Management Shell):
For mail-enabled public folders you can check identical parameters in the Public Folders Management Console or using Get-MailPublicFolder cmdlet output.