IMCEAEX NDR : Internal Users from Office 365

Whats going on?

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. 

Why is this going on?

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. 

IMCEAEX-_O=MMS_OU=EXCHANGE+20ADMINISTRATIVE+20GROUP+20+28FYDIBOHF23SPDLT+29_CN=RECIPIENTS_CN=User6ed4e168-addd-4b03-95f5-b9c9a421957358d@a6n.co.uk

How to fix it

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.