Hold down the Alt (Option) key while launching Outlook Express.
This should present you with the screen below. Click Yes to compact as normal.
If you click No, Outlook Express prompts you to carry out a complex rebuild.
NOTE: A complex rebuild is more thorough than the standard compacting procedure but is often less likely to work. Only try the complex rebuild on Outlook Express 5.02 and above (the previous versions might lose mail and address book data after a complex rebuild). The complex rebuild often requires a large block of memory to be assigned to Outlook Express. An insufficient memory allocation usually results in the rebuild quitting with Error Type 2. The chances of encountering such an error increase for large Identities folders.
The latest version of Outlook Express is available here
NOTE: A complex rebuild is much more likely to work if the Outlook Express Progress window is closed at launch. If you can launch Outlook Express normally, make sure that the Progress window is closed before quitting the application and trying the complex rebuild. If Outlook Express cannot be launched normally, it is recommended to move the Preferences and Schedules files out of the relevant Identities folder before trying the complex rebuild again.
Once the compacting has finished, you will be presented with the confirmation popup below. Click OK to open the main Outlook Express window.
NOTE: Despite what is written in the popup above, do not throw away the Old Database and Old Messages files until you are absolutely sure that Outlook Express is behaving correctly. Once you are happy that these files are no longer needed, remove them from the relevant folder within the Identities folder. On OS 9, the usual pathway for these files is Mac HD > Documents > Microsoft User Data > Identities > [Identity Name].
Below shows an example Identity folder after a standard compact of the Outlook Express database.
