This Sample is created with Visual Studio .Net 2003 Shared AddIn Wizard.
There are 2 Files: Connect.cs and XMailItem.cs.

------------------------------------------------------------------------Connect.cs --------------------------------------------------------------------------
// ##################################################################################
//
// Sample for Wrapping Mail Inspectors and creates a CommandBar, MenuItems
// by Helmut Obertanner ( flash [at] x4u dot de)
//
// ##################################################################################
namespace XConnect
{
	using System;
	using System.Collections ; 
	using System.Diagnostics ;
	using Microsoft.Office.Core;
	using Extensibility;
	using System.Runtime.InteropServices;
	using System.Windows.Forms; 

	// Note: When AddIn was created with Visual Studio Wizard, remove reference to Office.dll and
	// add reference to COM Office11 (Office 2003)
	using Ol = Microsoft.Office.Interop.Outlook ;  
	using MSO = Microsoft.Office.Core;  


	#region Read me for Add-in installation and setup information.
	// When run, the Add-in wizard prepared the registry for the Add-in.
	// At a later time, if the Add-in becomes unavailable for reasons such as:
	//   1) You moved this project to a computer other than which is was originally created on.
	//   2) You chose 'Yes' when presented with a message asking if you wish to remove the Add-in.
	//   3) Registry corruption.
	// you will need to re-register the Add-in by building the MyAddin21Setup project 
	// by right clicking the project in the Solution Explorer, then choosing install.
	#endregion
	
	/// <summary>
	///   The object for implementing an Add-in.
	/// </summary>
	/// <seealso class='IDTExtensibility2' />
	[GuidAttribute("9B73D13F-0476-4E16-86EF-A3A6B54EFFA8"), ProgId("XConnect.Connect")]
	public class AddIn : Object, Extensibility.IDTExtensibility2
	{

		/// <summary>
		/// The Outlook Application Object
		/// </summary>
		private Ol.Application myApplicationObject;

		/// <summary>
		/// My COM AddIn Instance
		/// </summary>
		private object myAddInInstance;


		/// <summary>
		/// The Inspectors Collection
		/// </summary>
		private Ol.InspectorsClass myInspectors;

		/// <summary>
		/// This Hashtable holds a reference to the active Inspectors
		/// </summary>
		private Hashtable myActiveItems = null;



		/// <summary>
		///		Implements the constructor for the Add-in object.
		///		Place your initialization code within this method.
		/// </summary>
		public AddIn()
		{
		}

		/// <summary>
		///      Implements the OnConnection method of the IDTExtensibility2 interface.
		///      Receives notification that the Add-in is being loaded.
		/// </summary>
		/// <param term='application'>
		///      Root object of the host application.
		/// </param>
		/// <param term='connectMode'>
		///      Describes how the Add-in is being loaded.
		/// </param>
		/// <param term='addInInst'>
		///      Object representing this Add-in.
		/// </param>
		/// <seealso class='IDTExtensibility2' />
		public void OnConnection(object application, Extensibility.ext_ConnectMode connectMode, object addInInst, ref System.Array custom)
		{
			// Get the initial Application object
			myApplicationObject = (Ol.Application) application;
			myAddInInstance = addInInst;

		}

		/// <summary>
		///     Implements the OnDisconnection method of the IDTExtensibility2 interface.
		///     Receives notification that the Add-in is being unloaded.
		/// </summary>
		/// <param term='disconnectMode'>
		///      Describes how the Add-in is being unloaded.
		/// </param>
		/// <param term='custom'>
		///      Array of parameters that are host application specific.
		/// </param>
		/// <seealso class='IDTExtensibility2' />
		public void OnDisconnection(Extensibility.ext_DisconnectMode disconnectMode, ref System.Array custom)
		{
			try
			{
				if (myInspectors != null)
				{
					// Unregister for NewInspector Event
					myInspectors.NewInspector -=new Microsoft.Office.Interop.Outlook.InspectorsEvents_NewInspectorEventHandler(myInspectors_NewInspector);

					// Release Reference to COM Object
					Marshal.ReleaseComObject (myInspectors);
				}

				myAddInInstance = null;

				// Release Outlook COM Object
				if (myApplicationObject != null) Marshal.ReleaseComObject (myApplicationObject);
				
				// Perform a Garbage Collection
				GC.Collect ();
			}
			catch (System.Exception ex)
			{
				MessageBox.Show (ex.Message);
			}			
		}

		/// <summary>
		///      Implements the OnAddInsUpdate method of the IDTExtensibility2 interface.
		///      Receives notification that the collection of Add-ins has changed.
		/// </summary>
		/// <param term='custom'>
		///      Array of parameters that are host application specific.
		/// </param>
		/// <seealso class='IDTExtensibility2' />
		public void OnAddInsUpdate(ref System.Array custom)
		{
		}

		/// <summary>
		///      Implements the OnStartupComplete method of the IDTExtensibility2 interface.
		///      Receives notification that the host application has completed loading.
		/// </summary>
		/// <param term='custom'>
		///      Array of parameters that are host application specific.
		/// </param>
		/// <seealso class='IDTExtensibility2' />
		public void OnStartupComplete(ref System.Array custom)
		{
			try
			{
				// Create a new Hashtable Object
				myActiveItems = new Hashtable (25);

				// Get the Outlook Inspectors Collection
				myInspectors = (Ol.InspectorsClass) myApplicationObject.Inspectors ;

				// Register for the NewInspector event
				myInspectors.NewInspector +=new Microsoft.Office.Interop.Outlook.InspectorsEvents_NewInspectorEventHandler(myInspectors_NewInspector);
		
			}
			catch (System.Exception ex)
			{
				MessageBox.Show (ex.Message);
			}			
		}

		/// <summary>
		///      Implements the OnBeginShutdown method of the IDTExtensibility2 interface.
		///      Receives notification that the host application is being unloaded.
		/// </summary>
		/// <param term='custom'>
		///      Array of parameters that are host application specific.
		/// </param>
		/// <seealso class='IDTExtensibility2' />
		public void OnBeginShutdown(ref System.Array custom)
		{
		}


		/// <summary>
		/// The eventhandler for the NewInspector event
		/// Called when an Item is opened
		/// </summary>
		/// <param name="Inspector"></param>
		private void myInspectors_NewInspector(Microsoft.Office.Interop.Outlook.Inspector Inspector)
		{
			try
			{
				object Item = Inspector.CurrentItem;
			
				// Check the ItemsType
				if (Item is Ol.MailItem)
				{
					// Create a new Item wrapper Object
					XMailItem myMail = new XMailItem (Item);

					// Register for the Item Close event
					myMail.Item_Closed +=new XEventHandler(myMail_Item_Closed);
					
					// remember the Item in Collection
					myActiveItems.Add (myMail.HashCode,myMail);

				}
			}						
			catch (System.Exception ex)
			{
				MessageBox.Show (ex.Message);
			}
		}

		/// <summary>
		/// Eventhandler for the Item close event
		/// </summary>
		/// <param name="sender"></param>
		/// <param name="e"></param>
		private void myMail_Item_Closed(object sender, XEventArgs e)
		{
			// Remove Item from Collection
			myActiveItems.Remove (e.HashCode );
		}
	}
}
-------------------------------------------- end of Connect.cs ------------------------------------------------------------------------------------------------------------
--------------------------------------------XMailItem.cs--------------------------------------------------------------------------------------------------------------------
// ##################################################################################
//
// Sample for Wrapping Mail Inspectors and creates a CommandBar, MenuItems
// by Helmut Obertanner ( flash [at] x4u dot de)
//
// ##################################################################################
using System;
using System.Diagnostics ;
using Microsoft.Office.Core;
using System.Runtime.InteropServices;
using System.Windows.Forms; 
using Ol = Microsoft.Office.Interop.Outlook ;  
using MSO = Microsoft.Office.Core;  

namespace XConnect
{

	/// <summary>
	/// EventArgs for my event
	/// </summary>
	public class XEventArgs : EventArgs
	{
		private readonly int myHashCode = 0;

		public int HashCode
		{
			get { return myHashCode; }
		}
		
		public XEventArgs(int hashCode)
		{
			myHashCode = hashCode;
		}
	}

	/// <summary>
	/// A delegate for my events
	/// </summary>
	public delegate void XEventHandler (object sender, XEventArgs e);



	/// <summary>
	/// This is a wrapper for a MailItemObject
	/// </summary>
	public class XMailItem
	{

		/// <summary>
		/// The "DATA"
		/// </summary>
		private Ol.MailItem myMailItem;

		/// <summary>
		/// the Inspector for the Item
		/// </summary>
		private Ol.InspectorClass myInspector;

		/// <summary>
		/// Event, raised when Item is closed
		/// </summary>
		public event XEventHandler Item_Closed;

		private int myID = 0;

		private MSO.CommandBar myCommandBar = null;
		private MSO.CommandBarComboBox myComboBox = null;
		private MSO.CommandBarButton myButton = null;

		private object myMissing = System.Reflection.Missing.Value ;  

		/// <summary>
		/// Returns the Hashcode for the Item
		/// </summary>
		public int HashCode
		{
			get { return myMailItem.GetHashCode(); }
		}
		
		/// <summary>
        /// The Constructor for the XMailItem
        /// </summary>
        /// <param name="Item">The Outlook MailItem Object</param>
		public XMailItem(object Item)
		{
			// Remember the Object
			myMailItem = (Ol.MailItem ) Item;

			myID = myMailItem.GetHashCode ();

			// Register for Item Open Event
			myMailItem.Open +=new Microsoft.Office.Interop.Outlook.ItemEvents_10_OpenEventHandler(myMailItem_Open);

			
		}


		/// <summary>
		/// Eventhadler for a Mail Open event.
		/// Remember this Item in ActiveItems
		/// Create Button here and register for button click events
		/// </summary>
		/// <param name="Cancel"></param>
		private void myMailItem_Open(ref bool Cancel)
		{

			// event isn't needed anymore
			myMailItem.Open -=new Microsoft.Office.Interop.Outlook.ItemEvents_10_OpenEventHandler(myMailItem_Open);

			// get the Inspector here
			myInspector = (Ol.InspectorClass ) myMailItem.GetInspector ;
			
			// register for the Inspector events
			myInspector.InspectorEvents_Event_Close +=new Microsoft.Office.Interop.Outlook.InspectorEvents_CloseEventHandler(myInspector_InspectorEvents_Event_Close);
			
			// Create the Menu
			CreateMenu();

		}

		/// <summary>
		/// Eventhandler for the INspector close event
		/// </summary>
		private void myInspector_InspectorEvents_Event_Close()
		{
			try
			{
				// Raise event, to remove us from active items collection
				if (Item_Closed != null)
				{
					Item_Closed(this, new XEventArgs (myMailItem.GetHashCode() ));
				}			
			
				// Cleanup resources
				myInspector.InspectorEvents_Event_Close -=new Microsoft.Office.Interop.Outlook.InspectorEvents_CloseEventHandler(myInspector_InspectorEvents_Event_Close);
				Marshal.ReleaseComObject (myInspector);
		
				Marshal.ReleaseComObject (myMailItem);
			}
			catch (System.Exception ex)
			{
				MessageBox.Show (ex.Message);
			}	
		}

		/// <summary>
		/// Adds the Menu for the MailItem
		/// </summary>
		private void CreateMenu()
		{
			try 
			{
				if (myInspector == null) return ;

				// Add our Own CommandBar to MailItem
				myCommandBar = myInspector.CommandBars.Add ("XConnect", myMissing, myMissing, true);
 
				// Add my ComboBox Control
				myComboBox = (MSO.CommandBarComboBox) myCommandBar.Controls.Add (MSO.MsoControlType.msoControlComboBox , myMissing, myMissing, 1, 1);

				myComboBox.Caption = "Company: ";
				myComboBox.Width = 270;

				myComboBox.Style = MSO.MsoComboStyle.msoComboLabel;  
					
				myComboBox.Tag = myID.ToString();

				// Fill the ComboBox
				FillComboFromDB();

				myComboBox.Change +=new _CommandBarComboBoxEvents_ChangeEventHandler(myComboBox_Change);


				// Add my button
				myButton = (MSO.CommandBarButton) myCommandBar.Controls.Add (MSO.MsoControlType.msoControlButton, myMissing, myMissing, 2, 1);  
				
				myButton.Caption = "XMail";
				myButton.Tag = myID.ToString();
				myButton.Style = MSO.MsoButtonStyle.msoButtonIconAndCaption;
				// Use one off 2 zilliards Office Icons....
				myButton.FaceId = 24;

				// Register for Click event
				myButton.Click +=new _CommandBarButtonEvents_ClickEventHandler(myButton_Click);
			
				// Make Menu Visible in TOP of Menus
				myCommandBar.Visible = true;
				myCommandBar.Enabled = true;
				myCommandBar.Position = MSO.MsoBarPosition.msoBarTop;  
			
			}
			catch (System.Exception ex)
			{
				MessageBox.Show (ex.Message);
			}
		}
		
		/// <summary>
		/// This function Fills the ComboBox from DataBase
		/// </summary>
		private void FillComboFromDB()
		{
			/// Simply a Jumpstart, not a complete solution
			/// Solutions costs money ;-)
			try
			{
				myComboBox.Clear ();
				
				// Should come from DB
				myComboBox.AddItem ("Foo",1);
				myComboBox.AddItem ("Bar",2);
				myComboBox.AddItem ("X4U",3);
				myComboBox.AddItem ("Slipstick",4);
				myComboBox.AddItem ("Turtleflock",5);
				myComboBox.AddItem ("DATALOG",6);

			}
			catch (System.Exception ex)
			{
				MessageBox.Show (ex.Message);
			}			
		}


		/// <summary>
		/// Cleans up any resources for shutdown
		/// </summary>
		private void CleanUp()
		{
			try
			{
				
			}
			catch (System.Exception ex)
			{
				MessageBox.Show (ex.Message);
			}
		}

		/// <summary>
		/// The ResetButton Click eventhandler
		/// </summary>
		/// <param name="Ctrl"></param>
		/// <param name="CancelDefault"></param>
		private void myButton_Click(CommandBarButton Ctrl, ref bool CancelDefault)
		{
			// Check, if we are in the right MailItem
			if (Ctrl.Tag != myID.ToString ()) return;

			myMailItem.Subject = "";	
		}

		/// <summary>
		/// Event comes, when Combobox has changed
		/// </summary>
		/// <param name="Ctrl"></param>
		private void myComboBox_Change(CommandBarComboBox Ctrl)
		{
			// Check, if we are in the right MailItem
			if (Ctrl.Tag != myID.ToString ()) return;

			CalculateMailSubject();
		}

		/// <summary>
		/// Creates the new Mail subject
		/// </summary>
		private void CalculateMailSubject()
		{
			try
			{
				string subject = "Learned from Company: " + myComboBox.Text + " @" + DateTime.Now.Hour + ":" + DateTime.Now.Minute ;
				
				// assign new subject to MailItem
				myMailItem.Subject = subject;
			}
			catch (System.Exception ex)
			{
				MessageBox.Show (ex.Message);
			}	
		}

	}
}
-----------------------------------------------------end of XMailItem.cs------------------------------------------------------------------------------

Free to use and modify to your requirements.

Greets, Helmut Obertanner