XADM: Duplicate E-Mail in Exchange 2000

Users on Exchange 2000 may erroneously receive multiple copies of messages. This behavior occurs in the following scenarios:

• If a user sends mail to a recipient that is a member of two distribution groups (formerly known as distribution lists, or DLs), and the distribution groups are expanded on different servers, recipients can experience duplicate mail delivery.
• If a user sends mail to a recipient that is a member of two distribution groups, and the distribution groups are in different Exchange organizations, recipients can experience duplicate mail delivery.

Duplicate detection on a recipient cannot occur if the recipient exists on bifurcated messages. Bifurcation (splitting of the original message) can occur for any of the following reasons:

• The original message requires content conversion; therefore, the message is bifurcated. This may occur when the recipients are contacts with specific content settings or when their mailboxes are on remote routing groups (other than the local routing group).
• The recipient is a distribution group with suppression settings or report settings other than "Report to Originator." That is, all of the following are true:
  • The Send OOF to the originator check box is cleared.
  • The distribution group is hidden.
  • The Report to Owner check box is selected.
• You send mail to a MAPI public folder or to application top-level hierarchy public folders.
• Journaling is enabled.

To summarize, if you send mail to a recipient and distribution group (with Out of Facility (OOF) messages turned off or if the Report to Owner check box is selected) that contains the recipient, that recipient gets mail twice.

Microsoft has confirmed that this is a problem in Microsoft Exchange 2000 Server.

Duplicate Detection in the Information Store

The information store detects duplicate messages based on the Internet Message ID; however, this works only if the duplicate e-mail delivery is attempted within one hour of the original mail delivery.

If the recipient is an Exchange 2000 mailbox, Exchange 2000 relies on the store's duplicate detection.

In Microsoft Exchange Server 5.5, the information store does not detect duplicate e-mail based on the Internet Message ID. Duplicate mail detection is based on the Instance ID. Unfortunately, this Instance ID is not copied over by the store when the Categorizer bifurcates a message. This problem exists in the Exchange Server 5.5 Internet Mail Service as well.

Therefore, if the recipient is a contact or Exchange Server 5.5 mailbox, the recipient always receives duplicate mail.

The information store uses a DeliveredTo table where, during local delivery, the table is updated to reflect the delivery of a unique message ID to a particular user.