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Luke_IT
Copper Contributor
Jun 13, 2023

LegacyDN with EXO (Microsoft 365)

Hi all,

  suddenly out of nowhere one of the Outlook clients in an office is unable to send an email to a M365 native recipient of the same org which never was migrated from onprem Exchange as this organization never had one (all the recipients were created as online since day 1) getting the usual ndr caused by an inexistant LegacyDN/X500 address. Nothing has been changed user side or org side. The auto-complete recipient was deleted from Outlook and rewritten but the error persists. By issuing a get-mailbox "user" | fl LegacyExchangeDN there is a LegacyDN though it's not the correct one (duh) so the Outlook client is clearly messing something up even after having deleted the autocomplete cache. Anyone had this issue?

Additionally, why would a native (not migrated from on prem and wih no AD Sync) EXO user have the LegacyDN attribute present if cloud native users should not be using this anymore? I think we've gone passed the point of backwards compatibility with Exchange 2007 - maybe I'm missing something but from what I recall the "new" EXO (internal) recipients should refer to email or proxy addresses and no longer to X500 as once was.

  • IMO, this warrants a support call as the legacyExchangeDN should not change. Unless something happened on the back-end. Make sure there is no IAM process that recreated the user/mailbox or another process that overwritten it; a (proper) restore w/reconnect (to mailbox) should not be a problem. And yes, behind the scenes, legacyExchangeDN is still used for internal resolution of recipients.

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