When using Exchange Anti-spam Recipient filtering agent with Recipient validation I have noticed that it stops the mail flow with NDRs for any messages sent to any recipient under any organization.
By default when having Exchange Anti-spam agents installed recipient filtering agent does not enable the Recipient validation feature however it gets enabled when forefront is being installed on your Hub servers and managing the exchange ant-spam agents.
To have your incoming mail flow working properly you have to disable this feature either from forefront console as per the below screenshot “Uncheck the only accept messages to valid recipients from the Global Address list check box”:
Or through Exchange PowerShell by using the following command.
Set-recipientfilterconfig –RecipientValidationEnabled $false