When you send email messages to an internal user in Microsoft Office 365 dedicated, you receive an IMCEAEX non-delivery report (NDR) because of a bad LegacyExchangeDN reference. The IMCEAEX NDR indicates that the user no longer exists in the environment.
This issue occurs because the value for the LegacyExchangeDN attribute changed. The auto-complete cache in Microsoft Outlook and in Microsoft Outlook Web App (OWA) uses the value of the LegacyExchangeDN attribute to route email messages internally. If the value changes, the delivery of email messages may fail with a 5.1.1 NDR. For example, the recipient address in the NDR resembles the following:
To resolve this issue, use one of the following methods, as appropriate for your situation.
Method 1: Clear the auto-complete cache file
For more information about how to clear the auto-complete cache file, click the following article number to view the article in the Microsoft Knowledge Base:
2005644 The Outlook auto-complete file contains obsolete or invalid entries
Note The procedure that's described here has to be performed by each user individually.
Method 2: Create an X500 proxy address for the old LegacyExchangeDN attribute for the user
To create an X500 proxy address for the old LegacyExchangeDN attribute for the user, make the following changes based on the recipient address in the NDR:
Replace any underscore character (_) with a slash character (/).
Replace "+20" with a blank space.
Replace "+28" with an opening parenthesis character.
Replace "+29" with a closing parenthesis character.
Delete the "IMCEAEX-" string.
Delete the "@mgd.domain.com" string.
Add "X500:" at the beginning.
After you make these changes, the proxy address for the example in the "Symptoms" section resembles the following:
X500:/O=MMS/OU=EXCHANGE ADMINISTRATIVE GROUP (FYDIBOHF23SPDLT)/CN=RECIPIENTS/CN=User-addd-4b03-95f5-b9c9a421957358d
Note The most common items will be replaced. However, there may be other symbols in the LegacyExchangeDN attribute that will also be changed from the way that they appear in the NDR. Generally, any character pattern of "+##" must be replaced with the corresponding ASCII symbol.