(Exchange) Admin Fix: X500 Address

Fixing X500 Address

Note: This document is intended for Exchange Administrators.

When you send email to someone who has a working Exchange mailbox and get an error like the one below, it usually means that the "legacyExchangeDN" attribute on the mailbox has changed.




 #550 5.1.1 RESOLVER.ADR.ExRecipNotFound; not found ##


There are several reasons that can happen, but the most common case is that the mailbox was deleted and a new one was created with the same email address. 

Outlook clients will try to send to the mailbox using the old legacyExchangeDN in the following cases:

  • Sending to a local contact stored for that person
  • Sending to an entry in the Outlook Nickname cache
  • Replying to an old email the person sent

Although it is possible to solve this problem by clearing the nickname cache and removing old contacts for all the senders who are effected, that is not really practical and replies to old emails will still bounce. The better solution is to add an X500 address to the new mailbox, so that mail can be properly routed.


To get the right syntax for the X500 address, use the text from the bounce message.

  • Remove the leading text "IMCEAEX-"
  • Change underscores to forward slashes
  • Change +20 to a space
  • Change +28 to an open parenthesis
  • Change +29 to a close parenthesis
  • Change +2E to a period


In the example above, the error provides this text:




The X500 address should be:




To add the X500 address to the affected account:

  • Open properties for the user account in Active Directory Users & Computers.
  • Choose the "E-mail Addresses" tab.
  • Click on "Add" and choose "Custom Address".
  • Enter the X500 address with the correct syntax per the steps above.
  • Choose "X500" for the "E-mail type".
  • Click OK twice to save.
  • Have one of the affected senders test sending to this account.