Problem:

When selecting the 'Out Of Office Assistant' from the Tools drop down menu in Outlook 2003 or 2007, you get an error message:- 'the command is not available. See the program documentation about how to use this extension'.

Solution:

There are 2 fixes – the first being preferred:

Fix 1 – within Outlook.

Open Outlook.
Click on Tools in Outlook 2003 or Help in Outlook 2007
Click on About Microsoft Outlook.
Click on the Disabled Items entry.
You will see a Disabled Items window open, with the following message on it:

“The items below were disabled because they prevented Outlook from functioning correctly.”
Select the ones you wish to re-enable.

If you see "Addin: outex.dll (outex.dll)" listed, click on it to highlight it and then click on the Enable button.

Then click on the Close button. Then click on the OK button to close the About Microsoft Office Outlook window. 

You may need to restart outlook to ensure the changes are implemented.

Fix 2 – Registry Edit (Not preferred)

Remove this registry key:

HKEY_CURRENT_USER\SOFTWARE\MICROSOFT\OFFICE\11.0\OUTLOOK\Resiliency