The error occurs when the following plug-ins are disabled or not installed in Outlook:
- Private Post Attachment Handler
- Private Post v4
To resolve the issue:
- On Outlook 2007, go to Tools > Trust Center > Add-ins.
- Select Exchange Client Extensions from the Manage dropdown list, and then click Go.
- Make sure that the following options in the Add-In Manager window are enabled:
- Private Post Attachment Handler
- Private Post v4
- Click OK.
- Restart Outlook.