Let postfix only accept mails for addresses in Exchange

I use postfix as a backup for my Exchange. I want postfix to only accept mail for recipients which exist in Exchange.

I assume you followed my first tutorial here

Existing postfix installation (mine is on debian)

Existing Exchange (2010 or 2013) installation

Create the following PowerShell script on your Exchange server:

$ExcludeUsers = @("Administrator", "user1")

Add-PSSnapin Microsoft.Exchange.Management.PowerShell.E2010

# Get all valid recipients, excludes DiscoverySearchMailbox and every user alias included in ExcludeUsers
$ValidRecipients = Get-Mailbox | where {$_.Alias -NotLike "DiscoverySearchMailbox*"} | where {$_.Alias -NotIn $ExcludeUsers}
foreach ($Address in $ValidRecipients.EmailAddresses)
{[string]::Format("{0}`tOK", $Address.AddressString)}

Run the script and save the output to a file like this: ./script.ps > output.txt

Connect to yout server running postfix

Upload the scriptoutput to /etc/postfix/relay_recipients

Add the following lines to your /etc/postfix/main.cf:

relay_recipient_maps = hash:/etc/postfix/relay_recipients
show_user_unknown_table_name = no

Reload or restart postfix

If your addresses change (added new or removed some) you have to repeat step 1 and 2