Office Outlook Error: the type or namespace name CreateItem does not exist namespace in ASP.Net

i need to open outlook from webapplication to send the mail with attachment and body content as html because we have to add address from outlook. i have to send mail with attachment using only outlook, no other services.

i have added outlook assembly

using Outlook = Microsoft.Office.Interop.Outlook;

still i am getting the error on below line Application.CreateItem.

the type or namespace name createitem does not exist namespace.