Here's the list of changes included in this rollup:
981820 New X-headers of a message item do not appear when the message item is retrieved by IMAP4 or by POP3 in an Exchange Server 2007 SP2 environment
2411423 The Msftefd.exe process constantly consumes up to 100 percent of CPU resources when your mailbox language is set to German on an Exchange Server 2007 server
2450078 The sent time in an email message body is incorrect when you reply or forward the email message by using an EWS application in an Exchange Server 2007 environment
2451415 “There was a problem logging onto your mail server” error message when you use a POP3 client to access a mailbox in an Exchange Server 2007 SP3 environment
2538958 Extended Protection Warning Displayed in Exchange Management Console and Exchange Management Shell After Installing RU2 for Exchange 2007 SP3
2554575 Items accumulate in the MRM submission folder when managed folder assistant journal items in an Exchange Server 2007 environment
2556751 The EdgeTransport.exe process crashes when processing certain email messages on an Exchange Server 2007 Hub Transport server
2557304 The Store.exe process may consume excessive CPU resources and memory resources intermittently when a user opens a calendar item by using OWA in an Exchange Server 2007 SP3 environment
When running ForeFront Protection for Exchange, make sure you disable ForeFront before installing the rollup and re-enable it afterwards, otherwise the Information Store and Transport services may not start. You can disable ForeFront using fscutility /disable and enable it using the fscutility /enable command.
Note that update rollups are cumulative, i.e. they contain fixes released in earlier update rollups for the same product level (RTM, SPx). This means you don't need to install previous update rollups during a fresh installation but can start with the latest rollup available right away.
You can download Exchange 2007 SP3 Rollup 5 here.