Every so often in Outlook Express it is worth running a few "maintenance checks". The main one is to run a compact to remove any wasted space. When you delete an email in Outlook Express, it is only marked as deleted and not physically deleted. These emails which are marked as deleted are what creates the wasted space. When you run the compact in Outlook Express this wasted space is removed.

First have a look at how much space is "wasted space".

Open Outlook Express and select "Tools" -> "Options" -> and then select the "Maintenance" tab.

On the Maintenance tab you have the button "Clean Up Now...", click this.

You are now presented with the "Local File Clean Up" window:

In the "File Information" section you will see you have 2 values:

Total size of file(s) = the total disk space being used

Wasted space = the amount of disk space in the Total size of Files which is wasted space and will be cleaned up with a compact.

Unfortunately in the above screen shot both are 0 bytes, however, if your Wasted Space has a value it is advisable to click the compact button.

Depending on the amount of space being used this could take a few minutes, just leave it to run.

Once the compact has finished click "Close"