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We are working on deploying Exchange 2010. We are new to this so I apologize if I'm missing something obvious.

We have had AD configured for some time now and are now gradually deploying Exchange 2010. We have had users receive the follow NDR:

From: Microsoft Outlook
Sent: Wednesday, April 20, 2011 4:42 PM
To: Jane Doe
Subject: Undeliverable: Quarterlies

 Delivery has failed to these recipients or groups:

 Jane Doe

 The e-mail address you entered couldn't be found. Please check the
 recipient's e-mail address and try to resend the message. If the problem
 continues, please contact your helpdesk.


  1. What exactly are these addresses? The user used auto-complete via Outlook and this is what it resulted in. Some searches lead me to believe it's something from Outlook's cached addres book? But where did it get it from? These are 2 completely new users in a new Exchange 2010 so it's not stale cache.

  2. How do we fix this?

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up vote 2 down vote accepted

quoted from:

"The problem is the cached Outlook nicknames are remember the information from the old Active Directory (Blue). Even though a user types in a fully qualified email address like outlook remembers used to be Outlook tries to simply take the old username from the old domain and put that in the new domain. So it tries to send to

Which makes the email bounce.

To resolve this issue:

We need to clear the Outlook nickname cache.

On XP/2000 the cache file is here:

C:\Documents and Settings\\Application Data\Microsoft\Outlook\.NK2

Close Outlook and delete the .NK2 file to get Outlook to forget previously entered information.

Under Vista it is here:


Close Outlook and delete the .NK2 file to get Outlook to forget previously entered information.

Deleting the .NK2 file only deletes the Outlook Cached addresses it does NOT delete the address book."

Hope this helps!

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How would this get cached? This is a new Exchange system. Or perhaps I'm not understanding this correctly. I will tell the user to try this out and see if it resolves the issue. – Belmin Fernandez Apr 25 '11 at 13:07

Alternatively set this as the x500 address - no user intervention required.

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I agree with Mike. The Outlook clients seem to keep the old addresses cached. But instead of deleting all cached addresses from your Outlook clients (internal and external), you may ask your users to:

  1. New mail
  2. Type in the first characters of the failed address
  3. Delete all entries of the failed address via 'DEL', scrolling up/down via the arrow keys
  4. Type in the address again or select it from the GAL via the 'to' field
  5. Send the mail.

Outlooks email address auto complete uses the address of the last successful delivery.

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To give you a step-by-step guide (for the solution which I think wooGZ refers to):

In the Exchange Management Console:

  1. open properties of the mailbox (or contact) which outlook cannot send to
  2. Inside the tab "E-Mail adresses", add another custom address:
    • Make sure to not have the type set to the default "SMTP" in the Add dialog.
    • I needed to choose "custom" in the drop down menu which was part of the "Add" button.
    • I've got a German version here, your labels may be a little different.
  3. in the type field of the Add dialog, enter X500
  4. in the address field of the Add dialog enter...


... or something very similar. If in doubt:

  • Either examine the "Legacy DN" property of other objects. (A disconnected mailbox in the Exchange management Console. Or in your Active Directory a user connected to a mailbox or maybe a contact. I did not choose this method.)
  • Or convert the string in the error(a.k.a. "NDR") E-Mail to a X500 address, as specified here by Microsoft. In short:

    Replace _ with /

    Replace +20 with a blank space.

    Replace +28 with (

    Replace "+29" with )

    Remove IMCEAEX-


  • It may be legal to prepend "X500:" in front of the address. I did not need it and I did not try.

  • the string in your question also has %-conversion in it, which complicates this process. Maybe you have access to a more original version without this additional obfuscation.
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