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SOLVED: Remote Server returned '420 4.2.0 Recipient deferred because there is no Mdb'

How to fix Non-delivery report error '420 4.2.0 Recipient deferred because there is no Mdb' by running Enable-RemoteMailbox PowerShell

Problem: Non-delivery report (NDR) error '4.2.0 Recipient deferred because there is no Mdb'

Non-delivery report error when trying to send emails to a user in an Office 365 - Exchange Hybrid environment

Delivery has failed to these recipients or groups:

Remote Server returned '420 4.2.0 Recipient deferred because there is no Mdb' 

Solution: Run the Enable-RemoteMailbox PowerShell command

This error is caused by the user not existing on the Exchange hybrid server. The mailbox is missing the user@domain.mail.onmicrosoft.com smtp address, which is used as the remote routing address to send emails to Office 365

To fix this, run the following PowerShell from the Exchange Hybrid server.

Enable-RemoteMailbox user@domain.com -RemoteRoutingAddress user@domain.mail.onmicrosoft.com 

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