If you’re not using an Edge server in your Exchange 2007 environment you can still add antispam features to your server.
In the Exchange installation folder (usually C:\Program Files\Microsoft\Exchange Server\) in the subfolder “Scripts” you will find the script “install-AntispamAgents.ps1”. Open the Exchange Management Shell, switch to the Scripts folder and run the script.
You will need to restart the Exchange Transport service and then you will find an extra tab “Antispam” in the Hub-Transport settings under Organization.
Adding zen.spamhaus.org to the block list providers significantly reduced the amount of spam I was receiving.