Exchange SP1 rollup 4 is highly recommended by anyone running SP1, but have a look at the KB article to make sure that it applies to you before diving ahead.......

See KB952580 for more detail

This update rollup does not apply to Exchange Server 2007 Release To Manufacturing (RTM). For a list of update rollups applicable to Exchange Server 2007 RTM, refer to the section Update rollups for Exchange Server 2007 RTM in KB937052.