XCON: X.400 MTA Logs "Loop Detected" Error Message When Receiving a Message from Exchange Server

In some cases, a third-party X.400 message transfer agent (MTA) may log an error message similar to "Loop detected" when it receives a message from Exchange Server. The originator of the message will receive a non-delivery report (NDR).

This behavior only occurs under very specific circumstances:

�	A message is sent from Exchange Server using the X.400 Connector. The recipients in the message are taken from the user's Personal Address Book (PAB) or are one-off addresses. At the same time, custom recipients exist for these addresses in the Exchange Server global address list.
�	The message may contain multiple trace records in the X.400 P1 message header, which are marked as "rerouted." This may cause the receiving MTA to think there is a message loop.
�	The number of trace records in the P1 message header depends on the number of recipients addressed. For example, if the message was sent to three recipients, there will be three trace records.

To resolve this problem, obtain the latest service pack for Exchange Server version 5.5. For additional information, click the following article number to view the article in the Microsoft Knowledge Base: The English version of this fix should have the following file attributes or later:

Component: Message Transfer Agent (MTA)

File name	Version
Dbserver.sch	N/A
Dcprods.cat	N/A
Ems_rid.dll	5.5.2621.0
Emsmta.exe	5.5.2621.0
Info4log.cfg	N/A
Infoblog.cfg	N/A
Infodlog.cfg	N/A
Infollog.cfg	N/A
Infoplog.cfg	N/A
Infotlog.cfg	N/A
Mtacheck.exe	5.5.2621.0
Mtamsg.dll	5.5.2621.0
P2.xv2	N/A
X400om.dll	5.5.2621.0
X400omv1.dll	5.5.2621.0

Microsoft has confirmed that this is a problem in Microsoft Exchange Server version 5.5. This problem was first corrected in Exchange Server 5.5 Service Pack 3.