Enabling Forefront Security Anti-spam Updates
An important factor in maintaining consistent spam protection over time is ensuring updates to the anti-spam filters are in place.
Microsoft Exchange Server 2007 leverages the proven Microsoft Update infrastructure for providing anti-spam filter updates and, by default, administrators can visit the MU site to retrieve content filter updates that are being published every two weeks.
An increased level of protection is offered to Enterprise CAL customers, where spam filter updates are more frequent and diverse http://www.microsoft.com/exchange/preview/edition_compare.mspx
Read more at source: http://msexchangeteam.com/archive/2007/01/03/432050.aspx