To troubleshoot and fix the 550 5.1.10 RESOLVER.ADR.RecipientNotFound issue within your Exchange environment, proceed with the following instructions:

1-  Launch the Exchange Management Shell (EMS).
2- Run the Get-RecipientFilterConfig cmdlet.

Get-RecipientFilterConfig

The output shows the RecipientValidationEnabled property as True.

3-  Navigate to the specific directory where the Exchange installation scripts are located using the command line. 

cd $env:ExchangeInstallPath\Scripts

    
4- Uninstall the anti-spam agents. This will remove the following anti-spam agents:

  • Content Filter Agent
  • Sender Id Agent
  • Sender Filter Agent
  • Recipient Filter Agent
  • Protocol Analysis Agent
.\uninstall-AntispamAgents.ps1

    
5- Disable the recipient validation.

Set-RecipientFilterConfig -RecipientValidationEnabled $false


    
6- Restart the Microsoft Exchange Transport Service.

Restart-Service MSExchangeTransport

    
7- Run the Get-RecipientFilterConfig cmdlet.

Get-RecipientFilterConfig

The output shows the RecipientValidationEnabled property as False.