Microsoft.Office.Tools.Outlook

This namespace contains a set of types that extend and support the Microsoft Outlook object model

Form Region

The only real difference between the VSTO 2005 Outlook 2003 add-in and the add-ins in VSTO 2005 SE is that the add-in host item base class wraps the Outlook Application PIA object
VSTO does not add any functionality to the Outlook object model
VSTO does not create any host items or host controls

Outlook Add-ins

These differ from Excel and Word add-ins because they must be registered in the registry
This means you must use a Setup Project to install your add-in.
When you create an Outlook add-in a setup project is automatically created for you ??
Outlook add-ins do not support Smart Tags, Task Pane Customisation or Data Caching.

Security

The security requirements of Outlook add-ins match those of Excel and Word document-level customisation.
By default VSTO solutions are very secure and require Code Access Security (CAS)
The security model in Outlook 2003 was improved with an email security update to help protect users from email viruses
This security model (often referred to as the Outlook Object Model Guard) prevents access to certain members of the Outlook object model.
It automatically trusts all installed COM Add-ins provided that you access the outlook objects through the Outlook application object (ThisApplication)
If you create a new instance then calls are restricted

Use ThisApplication instead of creating a new instance of an Application object

When Outlook is used with Exchange Server administrators can specify that all VSTO add-ins be trusted because they can specify that the AddinLoader.dll be trusted.

If macro security is set to VeryHigh or High then the add-in must be signed with a certificate
The AddinLoader.dll that shipped with VSTO was not signed, but installing VSTO 2005 SE will replace it with a signed version.

VSTO Add-ins (Outlook specific)

Visual Studio 2005 introduced the first VSTO type add-in which was only available for Outlook 2003.
This add-in model allowed Microsoft to get some valuable feedback before modifying the add-in model slightly before releasing Visual Studio 2005 SE.
The most significant change was to remove the host item base class (that wrapped the application PIA and made the add-in version specific) and replace it with an unwrapped application PIA object as a field in the class.
It is extremely easy to migrate an old style Outlook add-in to the new Outlook VSTO model.
The original VSTO 2005 Outlook add-in projects are still available in Visual Studio versions meaning that there are 2 VSTO add-in project types for Outlook 2003.

2003 add-ins will be registered the way that Office 2003 expects and uses VSTO runtime v2 even if it is running in Office 2007.