Problem: Recipient filtering is configured via the Exchange Management Console by opening Recipient Filtering and checking “Block messages sent to recipients that do not exist in the directory”, however all addresses are accepted with a “250 2.1.5 Recipient OK” message.

Solution 1: Run “Get-TransportAgent“.  If the output looks like this without the “Recipient Filter Agent”, then the antispam agents are not installed:

[PS] C:Windowssystem32>Get-TransportAgent
Identity Enabled Priority
——– ——- ——–
Transport Rule Agent True 1
Text Messaging Routing Agent True 2
Text Messaging Delivery Agent True 3

To fix this run “& $env:ExchangeInstallPathScriptsInstall-AntiSpamAgents.ps1” to install the agents and then restart the Exchange Transport service.  Now run “Get-TransportAgent” again and it should look like this:

[PS] C:Windowssystem32>Get-TransportAgent
Identity Enabled Priority
——– ——- ——–
Transport Rule Agent True 1
Text Messaging Routing Agent True 2
Text Messaging Delivery Agent True 3
Connection Filtering Agent True 4
Content Filter Agent True 5
Sender Id Agent True 6
Sender Filter Agent True 7
Recipient Filter Agent True 8
Protocol Analysis Agent True 9

Solution 2: Run “Get-AcceptedDomain | select Name,DomainType,AddressBookEnabled” and make sure that the Address Book is enabled on each of your domains.    If it’s not run “Set-AcceptedDomain yourdomain.com –AddressBookEnabled $true”

Solution 3: Check the receive connector and make sure the authentication is not set to Externally Secured or else all antispam agents will be bypassed.

Once this is fixed you will now see a “550 5.1.1 User unknown” message in the receive logs.