Have you checked that you haven’t any filters applied to this folder? Check out View \ Arrange By.
If you check the Sent Items folder in OWA, do you see the missing messages?
I had the same problem with Outlook. But when I checked my Sent Items in OWA, it had all of the items that I had sent.
Also, the problem magically stopped happening. I am missing most of a weeks worth of sent items, but the problem went away. Not sure yet what caused it.
I have also heard that this roll up patch for Exchange Server 2007 might help your cause as well.
Important The Exchange Server 2007 SP1 release notes address late-breaking issues that are not addressed in the documentation that is included with Exchange Server 2007 SP1. Before you install Exchange Server 2007 SP1, review the release notes for information that is pertinent to the system configuration.
To obtain the Exchange Server 2007 SP1 release notes, visit the following Microsoft Web site:
Exchange Server 2007 SP1 includes fixes that were previously released in the following update rollup packages for Exchange Server 2007. To view a list of issues that each update rollup fixes, click the article number to view the article in the Microsoft Knowledge Base:
941421 (http://support.microsoft.com/kb/941421/) Description of Update Rollup 5 for Exchange Server 2007
940006 (http://support.microsoft.com/kb/940006/) Description of Update Rollup 4 for Exchange Server 2007
935999 (http://support.microsoft.com/kb/935999/) Description of Update Rollup 3 for Exchange Server 2007
935490 (http://support.microsoft.com/kb/935490/) Description of Update Rollup 2 for Exchange 2007
930809 (http://support.microsoft.com/kb/930809/) Description of Update Rollup 1 for Exchange Server 2007
This service pack also fixes the following issues.
Note These fixes were not previously released in a rollup package.
939471 (http://support.microsoft.com/kb/939471/) The text of the e-mail message may be corrupted when Exchange 2007 receives an e-mail message that does not specify character set information
946641 (http://support.microsoft.com/kb/946641/) How to change the method for transfer encoding after you apply Exchange 2007 S946641 to the Exchange 2007-based server that is running the Hub Transport role