You have thousands of Offline Address Books (OABs) that are created on a Microsoft Exchange Server 2007 server. In this situation, the Microsoft Exchange File Distribution service (MSExchangeFDS.exe) occupies lots of memory. This behavior may affect the performance of other services. For example, the response to the client will be slower than expected.
This problem occurs because Exchange Server 2007 maintains the OAB list that includes thousands of objects in the memory. With additional OABs, the memory consumption of the MSExchangeFDS.exe process increases significantly.
To resolve this issue, install the following update rollup:
2407025 Description of Update Rollup 2 for Exchange Server 2007 Service Pack 3
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.