Where Jet Blue is not the same as Jet Red, the engine behind Access.
[link|http://www.microsoft.com/technet/prodtechnol/exchange/guides/E2k3TechRef/764d8347-a99b-408a-a774-f1263797c3b0.mspx?mfr=true|http://www.microsoft...3b0.mspx?mfr=true]
I think MS tried moving to SQL Server while deveoping some version of Exchange and gave up because of performance issues.