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This problem is not common but quite annoying and usually hard to understood by users.

In most cases problem occurs If you remove mailbox and re-create it with the same AD account , for an example after firing and  re-hiring of the employee after some time.

It also occurs after migrations.

When user sends an email to a recipient, to which earlier messages was delivered without any problem. Sudennly gets an Non Delivery Report with this content:

Delivery has failed to these recipients or distribution lists:: 

Test User

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..

Diagnostic information for administrators: 

Generating server:

#550 5.1.1 RESOLVER.ADR.ExRecipNotFound; not found ##

And now the question is: Why?

Mailbox exists , got a valid SMTP address, emails from other senders are delivered without any problems …

The trail leads to Outlook and auto completion of addresses. In the cache of sender’s Outlook  is stored a old recipient address. When addressing a message, Outlook automatically suggests the address to which was  previously sent messages.

For a internal addresses Outlook uses LegacyExchangeDN, and these addresses are stored in Outlook cache. And our recipient has the same SMTP addres , but different LegacyExchangeDN

What is the way out of this situation?

The easiest solution is to delete an old entry from Outlook suggested contacts. Re-selecting recipient  from GAL or manual typing address will save correct LegacyExchangeDN to Outlook cache. And the problem wil be solved.

Well, yes but this solution is a bit annoying for users. and, not really works well with a larger number of affected senders.

Second option is to add old legacyExchangeDN as a X500 address to the new mailbox.





In proactive approach we should write down and save this atribute. We can check this with folowing command:

Get-Mailbox user |fl legacyExchangeDN

But usually we do not have such information.

In this case, we can reconstruct the address from NDR.



And the X500 address will be look like:

/o=Company/ou=Exchange Administrative Group (FYDIBOHF23SPDLT)/cn=Recipients/cn=user
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