Either one Outbox.dbx or Sent Items.dbx file is damaged, so mail cannot automatically move to Sent Items from Outbox.dbx. Please delete it, the problem will be solved. It is because Outbox.dbx and Sent Items.dbx file is one of the system file, so the system will create the new one when this two system files cannot be found. Therefore, it will not affect the system when you delete them.

Before you delete this two files, please understand two main points as below
1. Under Windows XP, dbx file is hidden in Outlook Express.

We suggest you that turn on “Explorer display system file and hidden file” feature, and then you can see all of them. (Tools->Folder Option->View)

When you want to search dbx file, please tick “search system files, hidden files and files” in advanced searching.

Step:
there are two methods. If first one is not work, please use the second method.

Method 1:

Choose File -> Folder -> Compact all Folders

Method 2:

Open Outlook Express. Go to Tools -> Options -> Maintenance, then press “Store Folder”

Copy the path in the box, and then Close Outlook Express.

In the Taskbar, Go to Start -> Run.

Paste the Path to the box (The Path is copied at before), then press OK