There is a .NET CLR hotfix that has just been released and which is recommended to be applied on every Exchange 2007 server. It is more urgent to apply this on CAS servers with multiple protocols to reduce the memory load noticed on CAS servers with other roles, but in general it is recommended to be applied on all Exchange 2007 servers.

 

The KB article to reference is http://support.microsoft.com/default.aspx?scid=kb;EN-US;942027.