This depends on if you want the user to be able to see/edit the email in outlook before sending and if you want to see it in the users "sent" folder after they've sent it.

If no to both of these, then use CHILKAT.
If you want to populate an email which is presented to the user, then use a DLL wrapper around the Outlook .NET API
If you want to see an email in the users "SENT" folder, then use outlook 365 and the outlook REST API