protected void btnform_Click(object sender, EventArgs e)
  {
    MailMessage msg = new MailMessage();
    msg.From = new MailAddress("sirpuberto@gmail.com");
    msg.To.Add(email.Value);
    msg.Body = comentarios.Value;
    msg.IsBodyHtml = true;

    SmtpClient smtp = new SmtpClient();
    smtp.Host = "smtp.gmail.com";
    System.Net.NetworkCredential NetworkCred = new System.Net.NetworkCredential();
    NetworkCred.UserName = "sirpuberto@gmail.com";
    NetworkCred.Password = "quina9229946";
    smtp.UseDefaultCredentials = true;
    smtp.Credentials = NetworkCred;
    smtp.Port = 587 ;
    smtp.EnableSsl = true ;

    try
    {
        smtp.Send(msg);
    }

    catch (Exception ec)
    {

    }


}

<div class="seven_col_to_one_col">
        <form action="/contacto.aspx" method="post" runat="server" id="test">
            <input runat="server"  required name="nombre" type="text" class="form" id="nombre" placeholder="nombre"/>
            <input runat="server" required pattern="[^@]+@[^@]+.[a-zA-Z]{2,6}" name="email" type="email" class="form" id="email" placeholder="e-mail"/>
            <input runat="server" type="text" name="comentarios"  rows="8"  class="form comentarios" id="comentarios" placeholder="comentarios" />
            <button runat="server" id="submit" type="submit" class="btn_form" name="submit" value="enviar" onclick="btnform_Click">Enviar</button>
        </form>
    </div>