Allora, il codice funziona. Questo è quello che ho provato e la posta viene inviata e ricevuta
codice:
<%@ Page Language="C#" %>
<%@ Import Namespace="System" %>
<%@ Import Namespace="System.Net" %>
<%@ Import Namespace="System.Net.Mail" %>

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<script runat="server">
    protected void ButtoInvia_Click(object sender, EventArgs e)
    {
        string from = "***";
        //string to = "cataloghi@wecantour.it";
        string subject = "Richiesta Catalogo 2010 WeCanTour";

        string body = "<div>Invio dalla pagina Richiesta catalogo 2010 WeCanTour</div>";
        body = body + "<table width='36%' border='0' cellspacing='0' cellpadding='0'>";
        body = body + "<tr><td width='29%'>Nome:</td><td width='71%'>" + this.TextBoxNome.Text + "</td>";
        body = body + "</tr><tr><td>Cognome:</td><td>" + Cognome.Text + "</td></tr>";
        body = body + "<tr><td>Email:</td><td>" + TextBoxEmail.Text + "</td></tr>";
        body = body + "<tr><td>Ragione Sociale ADV:</td><td>" + TextBoxAzienda.Text + "</td></tr>";
        body = body + "<tr><td>Cellulare:</td><td>" + TextBoxCellulare.Text + "</td></tr>";
        body = body + "<tr><td>Telefono:</td><td>" + TextBoxTelefono.Text + "</td></tr>";
        body = body + "<tr><td>Fax:</td><td>" + TextBoxFax.Text + "</td></tr>";
        body = body + "<tr><td>Città:</td><td>" + TextBoxcitta.Text + "</td></tr>";
        body = body + "<tr><td>Indirizzo:</td><td>" + TextBoxindirizzo.Text + "</td></tr>";
        body = body + "<tr><td>CAP:</td><td>" + TextBoxCAP.Text + "</td></tr>";
        body = body + "<tr><td>Provincia:</td><td>" + TextBoxprovincia.Text + "</td></tr>";
        body = body + "<tr><td>Note:</td><td>" + TextBox2.Text + "</td></tr>";
        body = body + "</table>";
        //MailMessage EMAIL = new MailMessage(from, to, subject, body);
        MailMessage EMAIL = new MailMessage();
        EMAIL.From = new System.Net.Mail.MailAddress(from);
        EMAIL.To.Add(new System.Net.Mail.MailAddress("***"));
        EMAIL.To.Add(new System.Net.Mail.MailAddress("***"));
        EMAIL.To.Add(new System.Net.Mail.MailAddress("***"));
        EMAIL.Body = body;
        
        

        EMAIL.IsBodyHtml = true;
        EMAIL.Priority = MailPriority.High; // ALTA PRIORITA'

        System.Net.Mail.SmtpClient smtpMail = new SmtpClient("***");


        try
        {

            smtpMail.Send(EMAIL);

            this.TextBoxEmail.Text = "";
            this.TextBoxAzienda.Text = "";
            this.TextBoxCellulare.Text = "";
            this.TextBoxTelefono.Text = "";
            this.TextBoxFax.Text = "";
            this.TextBoxcitta.Text = "";
            this.TextBoxindirizzo.Text = "";
            this.TextBoxCAP.Text = "";
            this.TextBoxprovincia.Text = "";
            this.Cognome.Text = "";
            this.TextBox2.Text = "";
            this.TextBoxNome.Text = "";

            Label1.ForeColor = System.Drawing.Color.Black;
            Label1.Text = "La Sua richiesta è stata inviata con successo.";
            Label1.Visible = true;
        }
        catch
        {
            this.TextBoxEmail.Text = "";
            this.TextBoxAzienda.Text = "";
            this.TextBoxCellulare.Text = "";
            this.TextBoxTelefono.Text = "";
            this.TextBoxFax.Text = "";
            this.TextBoxcitta.Text = "";
            this.TextBoxindirizzo.Text = "";
            this.TextBoxCAP.Text = "";
            this.TextBoxprovincia.Text = "";
            this.Cognome.Text = "";
            this.TextBox2.Text = "";
            this.TextBoxNome.Text = "";
            Label1.ForeColor = System.Drawing.Color.Red;
            Label1.Text = "Errore durante l'invio del maessaggio";
            Label1.Visible = true;
        }

    }


</script>

<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <title></title>
</head>
<body>
    <form id="form1" runat="server">
    <div>
    
        <asp:TextBox ID="TextBoxNome" runat="server">azienda</asp:TextBox>
        

        <asp:TextBox ID="Cognome" runat="server">cognome</asp:TextBox>
        

        <asp:TextBox ID="TextBoxEmail" runat="server"></asp:TextBox>
        

        <asp:TextBox ID="TextBoxAzienda" runat="server"></asp:TextBox>
        

        <asp:TextBox ID="TextBoxCellulare" runat="server"></asp:TextBox>
        

        <asp:TextBox ID="TextBoxTelefono" runat="server"></asp:TextBox>
        

        <asp:TextBox ID="TextBoxFax" runat="server"></asp:TextBox>
        

        <asp:TextBox ID="TextBoxcitta" runat="server"></asp:TextBox>
        

        <asp:TextBox ID="TextBoxindirizzo" runat="server"></asp:TextBox>
        

        <asp:TextBox ID="TextBoxCAP" runat="server"></asp:TextBox>
        

        <asp:TextBox ID="TextBoxprovincia" runat="server"></asp:TextBox>
        

        <asp:TextBox ID="TextBox2" runat="server"></asp:TextBox>
        

        

        <asp:Label ID="Label1" runat="server" Text="Label"></asp:Label>
        

        

        <asp:Button ID="Button1" runat="server" onclick="ButtoInvia_Click" Text="Button" />
        

    
    </div>
    </form>
</body>
</html>

ps. al posto degli asterischi ci vanno naturalmente gli indirizzi di posta