If you had to change the legacyExchangeDN for your users as part of a migration or other manual process, or even an error in your automated user provisioning software, there are certain things that you need to be aware of.
For one thing, cached Outlook ‘autocomplete’ entries will stop working. If a sender uses his Outlook autocomplete to select a user whose legacyExchangeDN has been recently changedand sends an email to a user, chances are that the mail will bounce with the following NDR:
The recipient’s e-mail address was not found in the recipient’s e-mail system. Microsoft Exchange will not try to redeliver this message for you. Please check the e-mail address and try resending this message, or provide the following diagnostic text to your system administrator.
The first thing you need to do after changing the legacyExchangeDN is to initiate an Offline Address Book rebuild. The way out of this problem is to instruct the sender to first download the Address Book by doing a full Send/Receive. Then ask the sender to manually select the affected recipient from the Address book instead of using the cached Autocomplete.
This is because, in an Exchange organization, Outlook client autocomplete caches do not save the SMTP email address. Instead, it saves the X.500 address using the legacyExchangeDN attribute from Active Directory. And that’s exactly what you have changed!
There is another workaround to prevent the NDRs. This would be to enter the old value of the legacyExchangeDN attribute as an X.500 address for the user account. This is done by opening the user object’s Email Address properties –> Add a Custom Address –> Enter the old value of legacyExchangeDN as the address and the address type as "X500" without the quotes. The drawback of this method is that you are populating the user’s email address field with garbage, which you don’t need after a while but can’t remove because you are unsure if traces of the old entries in Outlook caches are gone!