<system.net>
    <mailSettings>

<smtp>
        <network host="smtpserver" port="25" userName="uid" password="pwd" />
      </smtp>

</mailSettings>
  </system.net>

<table border="0">
        <tr>
            <td>
                发件人
            </td>
            <td>
                <asp:TextBox runat="server" ID="emailfrom"></asp:TextBox>
            </td>
        </tr>
        <tr>
            <td>
                收件人
            </td>
            <td>
                <asp:TextBox runat="server" ID="emailto"></asp:TextBox>
            </td>
        </tr>
        <tr>
            <td>
                主题
            </td>
            <td>
                <asp:TextBox runat="server" ID="subject"></asp:TextBox>
            </td>
        </tr>
        <tr>
            <td>
                附件
            </td>
            <td>
                <asp:FileUpload ID="attachment" runat="server" />
            </td>
        </tr>
        <tr>
            <td>
                内容
            </td>
            <td>
                <asp:TextBox runat="server" ID="body" TextMode="MultiLine" Columns="50" Rows="10"></asp:TextBox>
            </td>
        </tr>
        <tr>
            <td colspan="2" align="center">
                <asp:Button runat="server" ID="btnSend" Text="发送" OnClick="btnSend_Click" />
            </td>
        </tr>
    </table>

SmtpClient sc = new SmtpClient();

// 编程方式设置smtp（不用web.config）
        // sc.Host = "";
        // sc.Port = 25;
        // sc.Credentials = new NetworkCredential("username", "password");

try 
        {
            sc.Send(mm);
            Response.Write("ok");
        }
        catch (Exception ex) 
        {
            // 与smtp相关的错误
            if (ex is SmtpException) 
            {
                // ex.ToString();
                Response.Write("smtp发信失败");
            }
            else 
            {
                Response.Write(ex.ToString());
            }
        }