Enable Recipient Filtering for Exchange 2013/2016/2019

Recipient filtering is a feature in Exchange Server that relies on the RCPT TO SMTP header to determine what action, if any, to take on an inbound message. Recipient filtering is performed by the Recipient Filter agent which is part of the Anti Spam Agents package. These agents are enabled by default on Edge Transport servers, however you need to enable them manually on Exchange Mailbox servers.

1. Open the Exchange Management Shell on the Exchange Mailbox server.
2. Enter the command:

   Get-TransportAgent

3. If "Recipient Filter Agent" does not appear in the list, install the anti-spam agents:

   & $env:ExchangeInstallPath\Scripts\Install-AntiSpamAgents.ps1

4. Make sure the Recipient Filter Agent agent is enabled:

   Enable-TransportAgent "Recipient Filter Agent"

5. Disable all other anti-spam agents:

   Disable-TransportAgent "Content Filter Agent"
   Disable-TransportAgent "Sender Id Agent"
   Disable-TransportAgent "Sender Filter Agent"
   Disable-TransportAgent "Protocol Analysis Agent"

6. Ensure your accepted domains are using Address Book to check for valid recipients. By default, this should be enabled when Exchange is an authoritative Mailbox Server for the domain. To determine if the Address Book is enabled, run the following command:

   Get-AcceptedDomain | Format-List Name,AddressBookEnabled

7. If the Address Book is disabled, use the following command replacing example.com with your domain:

   Set-AcceptedDomain example.com -AddressBookEnabled $true

8. Enable recipient validation

   Set-RecipientFilterConfig -RecipientValidationEnabled $true

9.  Restart the "Microsoft Exchange Transport" service:

   Restart-Service MSExchangeTransport