This depends on the way the MX record is setup. In most cases, the Backup mail server should be set up as the secondary priority and, as a result, most of the emails would be delivered to your Exchange Server first. All of our MX Backup Solutions come with anti-spam and virus filtering during bagging time only.  However, you may opt to utilize continuous anti-spam and virus filtering. The Anti-Spam portals are located at multiple premium facilities in East and West coast. This solution will route emails through the anti-spam servers before sending them to your Exchange server during your uptime.  During primary mail server down time, emails pass through the filtering system and are then sent to our MX Backup Solution where they are held until your primary mail server becomes available.

For more information regarding our anti-spam and virus filtering services, please view:

Postini Anti-Spam and Virus Filtering
SpamWeeder Anti-Spam and Virus Filtering