Exchange

Occasionally in Exchange you will accidentally or intentionally recreate a user, or delete some values from their object - this will impact the LegacyExchangeDN value. I have seen this happen where mailuser objects were removed from Office 365, the user objects were cloud only and the LegacyExchangeDN was gone forever. Contrary to its name, the LegacyExchangeDN value is critical to maintaining active conversations within your organisation. When a user’s legacyExchagneDN is lost, collegues replying to conversations with that user will get an NDR.