Unable to deliver NDR because of Bounce verification

Hi ArjanBroekhuizen,

To make sure that Non-Delivery Reports (NDRs) are successfully delivered to the end-user mailbox in Exchange Online (Office 365), you can try to use these options (steps):

1. Make sure that the external system sending NDRs uses a valid email address that can be accepted by Exchange Online.

Email non-delivery report (NDR) and SMTP errors in Exchange Online - Exchange | Microsoft Learn

2. turn off the Directory Based Edge Blocking (DBEB) feature for your verified domain in Exchange Online. DBEB is a security measure that can sometimes mistakenly block legitimate NDRs. 

To disable DBEB, you can use the following PowerShell command:

Set-RemoteDomain -Identity "Default" -UseDirectoryEdgeBlockList $false

Use Directory-Based Edge Blocking to reject messages sent to invalid recipients in Exchange Online | Microsoft Learn

3. Configure a transport rule in Exchange Online to redirect NDRs to the end-user mailbox. 

To create the transport rule, you can use the following PowerShell command:

New-TransportRule -Name "Redirect NDRs to End-User Mailbox" -RecipientDomainIs "yourdomain.com" -MessageTypeMatches "NDR" -RedirectMessageTo "email address removed for privacy reasons"

*Replace "yourdomain.com" with your actual domain and "email address removed for privacy reasons" with the email address of the end-user.

Mail flow rules (transport rules) in Exchange Online | Microsoft Learn