Scalable Networking Pack (SNP) hotfix for Windows 2003
Scalable Networking Pack (SNP) is a feature that was introduced with Windows Server 2003 and that also exists in Windows Server 2008 (but it's turned off by default).
Although SNP was a good idea, as it could improve network performance, it may have some adverse effects on Exchange (for more details, read Windows 2003 Scalable Networking pack and its possible effects on Exchange).
After the report of these problems, Microsoft released a hotfix that would disable SNP features. A new hotfix has now been released, if you installed the first one, I strongly advise you to go and get this new one.