The e-mall address you entered couldn’t be found Exchange

Hey there,

Recently, I had to move my customer from Microsoft Exchange 2010 to office 365 services, due to financial circumstances the customer decided to return back to the on-premises environment.

But while returning back we encounter in several unexpected issues with sending emails from some end-users.

When a user tries to send an email to certain mailbox we receive the following error:

The e-mail address you entered couldn’t be found. Please check the recipient’s e-mail address and try to resend the message. If the problem continues, please contact your helpdesk.
550 5.1.1 RESOLVER.ADR.ExRecipNotFound; not found ##

I wasn’t able to realize why it happens, after a strong headache, I noticed that the “LegacyExchangeDN” was wrong, and consequently would have to act accordingly¬†ūüôā

  1. Open the Active Directory and search the users you are looking for.
  2. Right Click and properties
  3. Attribute Editor
  4. find the “LegacyExchangeDN
  5. in this attribute, I found this “IMCEAEX-_O=FIRST+20ORGANIZATION_OU=EXCHANGE+20ADMINISTRATIVE
    +20GROUP+20+28FYDIBOHF23SPDLT+29_CN=RECIPIENTS_CN=f3f12dk0kg044g4g0003fMeirp@pelegit.co.il”

As you can figure out the “CN” part is wrong, the solution is to delete\modify the¬†RECIPIENTS_CN’s value with the correct user’s common name.
In some cases, the LegacyExchangeDN¬†value changed and you should create “X500 proxy address for the old LegacyExchangeDN” with the same CN you get in the NDR “[email protected]”, otherwise, users receive endless NDR messages, it’s up to you, you can add the¬†X500 proxy address and in-parallel modify the old one and in fact you solve the issue thoroughly.

For further information:
https://support.microsoft.com/en-au/help/2807779/imceaex-non-delivery-report-when-you-send-email-messages-to-an-interna