Enable RecipientValidation in Exchange Organisation
I have been troubleshooting an issue where mail was reaching our Exchange servers for non-existent recipients (it passes through a Sophos UTM first). I discovered that there is a way of having Exchange validate the recipient and this is done using the Set-RecipientFilterConfig cmdlet, setting the RecipientValidationEnabled parameter to true:
Set-RecipientFilterConfig -RecipientValidationEnabled $true
I can now check the config using the Get-RecipientFilterConfig cmdlet: