Assume that daylight saving time (DST) is implemented at midnight in a region. Some Microsoft Exchange Server 2007 or Exchange Server 2010 users in the region open recurring calendar items in mailboxes by using Outlook Web App (OWA) or an Exchange Web Services (EWS) application. In this situation, you may find that the W3wp.exe process in the MSExchangeServicesAppPool application pool consumes 100 percent of CPU resources on the Client Access servers. When this issue occurs, OWA or the EWS application cannot connect to the Exchange Server servers.
This issue occurs because the next occurrences of the calendar items are calculated incorrectly after DST is implemented. Additionally, the start dates for the calendar items become invalid. Therefore, when OWA or EWS tries to obtain the next occurrences of the calendar items, an infinite loop occurs.
To resolve this issue, install the following update rollup:
For Exchange Server 2010
2661854 Description of Update Rollup 2 for Exchange Server 2010 Service Pack 2
For Exchange Server 2007
2873746 Description of Update Rollup 11 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.