replace the line
Outlook.MailItem mailItem = (Outlook.MailItem)
this.Application.CreateItem(Outlook.OlItemType.olMailItem);
with
Microsoft.Office.Interop.Outlook.Application app = new Microsoft.Office.Interop.Outlook.Application();
Microsoft.Office.Interop.Outlook.MailItem mailItem = app.CreateItem(Microsoft.Office.Interop.Outlook.OlItemType.olMailItem);
Hope this helps,