You create a mail-enabled user that has an external email address in a Microsoft Exchange Server 2010 Service Pack 1 (SP1) environment. You try to add the external email address to the safe sender list in Outlook Web Access (OWA). In this situation, you receive the following error message:
This operation fails with the following error Property validation failed. Property = TrustedSendersAndDomains (System.String) Error = "<external e-mail address>" is in your organization and can't be added to Safe Senders and Recipients.
This issue occurs because the external email address is treated as an internal email address.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.