Outlook Express automatically compacts messages

With XP Service Pack 2 Microsoft made a number of changes to how Outlook Express compacts your emails. As with any storage in Windows (mail, database etc) when you delete data it is only marked as deleted and not fully removed. Running compacting tools will delete this data and free up space.

In Outlook Express the compacting starts after you have opened and closed Outlook Express 100 times. Unfortunately this value cannot be increased or decreased within the software

In early version of Windows XP,to resolve this behavior, turn off background compaction:

1. In Outlook Express, on the Tools menu, click Options.
2. Click the Maintenance tab, click to clear the Compact messages in the background check box, and then click OK.
3. Quit and then restart Outlook Express.

Windows XP Service Pack 2 to reset it manually:

1. Close Outlook Express and all message windows.
2. Click Start, click Run and type without quotes "regedit".
3. Navigate to this key
   HKEY_CURRENT_USER\Identities\{GUID}\Software\Microsoft\Outlook Express\5.0
   where {GUID} is the long number used for your Identity.
4. In the right-hand pane, double-click "Compact Check Count", then change the value to 0.