This is from the Microsoft Knowledge Base:
OLEXP: MAPI Error Messages When You Import or Export Messages
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SYMPTOMS
When you try to import messages into Outlook Express, you may receive the following error message:
Messages can not be imported from the MAPI client. An error has occurred.
When you try to export messages from Outlook Express, you may receive the following error message:
The Export could not be performed. An error occurred while initializing MAPI.
CAUSE
This behavior can occur if you configure Outlook Express to be the default Simple Messaging application programming interface (MAPI) client.
RESOLUTION
To work around this issue, follow these steps:
Restart your computer, and then start Outlook Express.
On the Tools menu, click Options.
On the General tab, click the "Make Outlook Express my default Simple MAPI client" check box to clear it, and then click Apply.
Click OK when you receive the following prompt:
The Simple MAPI client has been changed. You may need to restart your machine for the change to take effect.
Click OK, quit Outlook Express, and then restart your computer. You should be able to import or export messages.
NOTE: After you complete the import or export process, you can configure Outlook Express to be the default MAPI client. For information about how to configure Outlook Express to be the default MAPI client, see the following article in the Microsoft Knowledge Base: