Contact form not sending

Hello
What might be the reason, please, why my contact form is not working? When I click 'Send' I don't get any server messages, but I should be directed to a 'Thank you' page and I am not. The form remains on screen with blank fields.
I am using the same smtpserver name and password that I use for a Classic ASP contact page that I have just tested and it works. That would rule out, I think, a password or server problem. That is, the issue is probably with my code. Or the Web.config file. There is no Captcha to complicate the problem.
The relevant code, stripped down, in my aspx file looks like this: 
<asp:TextBox ID="your_name" runat="server" required></asp:TextBox></p> <asp:TextBox ID="your_email" runat="server" required></asp:TextBox></p><br /> <textarea id="your_message" runat="server" cols="50& ...