If you updated your server to Exchange Server 2007 SP1 Rollup Update 3 then you will be able to remove the text “Sent by Microsoft Exchange Server 2007” using the following cmdlet:
Set-TransportConfig WritingBrandingInDSNEnabled $false
More information at: http://support.microsoft.com/default.aspx?scid=kb;en-us;941770&sd=rss&spid=10926