By default, NDR (5.4.4) of an email sent to a user with non-existent domain is sent to postmaster's mailbox. If you want your users to get NDR when they send email to a wrong email address (domain in this case), you need to remove 5.4.4 code from Transport Configuration. In Exchange 2010: go to Organization > Hub transport > Global settings > Transport settings remove code 5.4.4 from the list In Exchange 2013/16 Get-transportConfig to check settings Find GenerateCopyOfDSNFor and the codes (if 5.4.4 is on the list) Example: {5.4.8, 5.4.4 , 5.4.6, 5.2.4, 5.2.0, 5.1.4} Copy ALL codes (from bracket to bracket) Paste into Notepad Remove 5.4.4 from the list (remove 5.4.4, ) Set-transportConfig -GenerateCopyOfDSNFor "5.4.8, 5.4.6, 5.2.4, 5.2.0, 5.1.4" Users will get NDR immediately.
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