Email Form in C# - not sending

Hi there,
I'm trying to write a simple email form page - my code is below.
When pressing the send button I get an error - "The "SendUsing" configuration value is invalid."
Also MailMessage is underlined in green.
Could anyone please help with this.
Many thanks in advance.
The code:



using System; using System.Collections.Generic; using System.Linq; using System.Web;

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