Normally, this means that your data stores have gotten too big. You can check the server's Application log which should log an error, Event 9685 from the MSExchangeIs Mailbox Store. The error text will tell the size of the data stores.
If this is the case, there's a Microsoft Fix-It for that; accessible below:
Exchange 2003 Services have to be restart daily
Posted - September 06 2011 : 13:51:08
