Many of the fixes in Outlook 2003 SP3 were previously released in updates or hotfixes. New features are all security-related and most can be disabled by editing a registry key, if necessary for your business. Office 2003 SP3 combines previously released fixes into one update so you only need to install one service pack if you have to reinstall Office 2003, not each service pack in succession.
Included in the security fixes are the following items:
Office 2003 can no longer open or save certain file formats. This is not an Outlook issue, as the file types are not blocked in email messages; however I’m mentioning it because it has the potential to affect many sites since Office 2003 applications are blocked from opening files created in older file formats. See KB article 938810 for more information. http://support.microsoft.com/kb/938810/
Outlook 2003 SP3 (and Outlook 2007) limits some of the ways that custom properties can be introduced into Outlook data stores. By default, Outlook no longer enables Internet mail to create new custom properties. Only properties that are already created in the default mail delivery store are preserved for incoming e-mail messages. This means opening a template (.oft) from the file system may fail with the error "The custom form could not be opened. An Outlook form will be used instead". This can be disabled with a registry key. See KB article 907985 for the key needed to disable this feature.
MAPI forms do not run in public folders and user folders because by default, SP3 blocks MAPI forms that cause new executables to be downloaded and started. KB article 938816 has more information on this security feature and instructions on how to disable it if your site is affected.
Office 2003 can be configured to allow or deny specific COM components. This feature lets administrators and users set registry keys in order to enable or to disable the instantiation point of a specific COM category in an Office application. See KB 938815 for instructions.
The behavior of add-ins, ActiveX controls, and COM add-ins was changed in some scenarios in Office 2003 SP3 to improve the security poorly written COM components. This can be disabled in the registry but the recommendation is for the developer of the add-in or control update their application. See KB 938814 for more information.
Attachments with the .gadget extension can no longer be opened in Outlook. See KB 938811 for more information. Ken Slovak has updated his Attachment Options application to allow users to manage this and other blocked extensions.
Issues that are fixed in Outlook 2003 by Office 2003 Service Pack 3
Microsoft has an Excel workbook available for download which contains a complete list of issues fixed in this service pack.
How to obtain the latest service pack for Office 2003
Microsoft Office 2003 SP3 Breaks Outlook Forms (.oft)