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Event ID 9031 issue when deleting Exchange Server mailboxes

If an Exchange Server mailbox is deleted and recreated by hand, the LegacyExchangeDN property in Active Directory will not be the same between the two versions of the mailboxes. This can create email delivery issues and generate Event ID 9031 errors.

When an Exchange Server user mailbox account is mail-activated, the LegacyExchangeDN property is created in Active...


This property uniquely identifies the mailbox from all others in Microsoft Exchange -- rather than being identified by the mailbox name or username. LegacyExchangeDN provides backward compatibility with Exchange 5.5 as well.

An AD object that is not mail-activated will have a LegacyExchangeDN property, but with no value set.

If an Exchange Server mailbox is deleted and recreated by hand, the LegacyExchangeDN property will not be the same between the two versions of the mailboxes. Normally this isn't a problem, especially for email that arrives from the Internet -- Exchange Server will handle that transparently.

But issues can arise when someone within the organization, sending email directly through Exchange Server, tries to send a message to the old mailbox. When the email is processed, it generates an error with Event ID 9031 (for MSExchangeTransport/Categorizer).

There are a couple of possible scenarios that can cause this:

  1. A Microsoft Outlook user has an entry for that Exchange Server mailbox in his Contacts, but it uses the old mailbox data because it hasn't been updated.

  2. Someone is replying to an older email message in Exchange Server that contains the outdated mailbox information.

If you believe there will be many future attempts to reply to an old mailbox address -- for instance, if there are still several unresponded to emails for that user account floating around -- an easy workaround is to add an X.500 custom address to the new mailbox, and use the old LegacyExchangeDN value as the address. This value should show up in the data of the error message as a string with the header ex:/.

About the author: Serdar Yegulalp is editor of the Windows Power Users Newsletter.

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