Symptoms

User tries to send an attachment to Outlook.   They receive an error dialog as follows:

Unable to send files to target. This object model command is not available in email.

Resolution

This issue can be caused by either a problem with an Outlook addin that the user may have installed, or a corrupted Outlook installation.

Checking for a problem Outlook addin

1. Close Outlook
2. Hold the Control key down
3. Launch Outlook (while still holding the control key down)
4. Outlook will launch in Safe Mode
5. After Outlook is open, test your attachment.   If the problem no longer occurs, then there is an Outlook add-in that is causing problems.
6. Remove the problem Outlook addin (either remember what software you recently installed and uninstall it, or use the Outlook COM add-in manager to disable add-ins until you find the culprit).

Repairing an Outlook installation

Outlook 2003: Use Help>Detect and Repair
Outlook 2007: Help>Office Diagnostics
Outlook 2010: Use Control Panel, Programs and Features, select Microsoft Outlook, click Repair
Outlook 2013: Use Control Panel, Programs and Features, select Microsoft Outlook, click Repair