You have two tenant organizations (Organization 1 and Organization 2) in a multi-tenant Microsoft Exchange Server 2010 environment. The environment has Exchange Server 2010 Service Pack 1 (SP1) or a later update rollup installed.
Organization 1 has two mail-enabled users (User A and User B).
Organization 2 has a distribution group that contains User A.
User B in Organization 1 tries to send an email message to the distribution group that is in Organization 2.
In this scenario, User B receives a non-delivery report (NDR) message that contains the following:
550 5.7.1 Transport rules loop count exceeded
This issue occurs because the x-ms-exchange-transport-rules-loop value that is used to track message looping is set in the message header incorrectly when the email message is sent between tenant organizations.
To resolve this issue, install the following update rollup:
2706690 Description of Update Rollup 4 for Exchange Server 2010 Service Pack 2
To work around this issue, create an external contact for User A in Organization 2, and then add the contact to the distribution group.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
For more information about the multi-tenant Exchange deployment, go to the following Microsoft website: