Recently while troubleshooting a customer’s mail flow rules to prevent their Exchange Online Protection from quarantining Automatic Replies, I came across a rule that they had created to prevent the system from sending NDRs to spam bots and attackers trying to guess email addresses on their system. Their rule wasn’t configured properly, so it was quarantining their own auto reply emails to external senders.
A simple way to achieve the actual desired result is to just disable NDRs on either a specific domain, or all domains.
Open an Exchange PowerShell and issue the following command:
Set-RemoteDomain <domain> -NDREnabled $false
If you’d like to turn off ALL NDRs, issue this command:
Set-RemoteDomain * -NDREnabled $false
Here is a TechNet article that details this command in more detail.
Joe Hanning, PEI