Event ID - 4123

Event Id4123
SourceMSExchangeIMC
DescriptionA message passing through the Internet Mail Service has been intentionally dropped. This is most likely an admin notification (message describing mail failure), an out of office notification (OOF),or an automatic reply from a users mailbox. Dropping of OOF messages and automatic replies is configurable in the admin pages. The gateway must always drop notifications to the administrator, since they often cause mail loops.
Event InformationA third-party gateway may add the PR_AUTO_FORWARDED property per documentation in the Microsoft Developer Network (MSDN). The message is delivered into the Exchange system. If the message is replied to by a user on Exchange and sent to an address that will be delivered through the Internet Mail Service, the system acts on the message based on the propertys existence and not its value.
The WorldTalk gateway may add the PR_AUTO_FORWARDED property to messages.
RESOLUTION
To resolve this problem, obtain the latest service pack for Exchange Server version 5.5.
Reference LinksEvent 4123; Message Not Delivered When PR_AUTO_FORWARDED & Disable Auto Replies Set

XWEB: Autoforwarded Messages Cannot be Forwarded to External Addresses

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