Sending an email with asp.net

(1)Sending Email with normal text along with attachment
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(i)UI page
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 <table border="0" cellpadding="0" cellspacing="0">
    <tr>
        <td style="width: 80px">
            To:
        </td>
        <td>
            <asp:TextBox ID="txtTo" runat="server"></asp:TextBox>
        </td>
    </tr>
    <tr>
        <td>
            &nbsp;
        </td>
    </tr>
    <tr>
        <td>
            Subject:
        </td>
        <td>
            <asp:TextBox ID="txtSubject" runat="server"></asp:TextBox>
        </td>
    </tr>
    <tr>
        <td>
            &nbsp;
        </td>
    </tr>
    <tr>
        <td valign = "top">
            Body:
        </td>
        <td>
            <asp:TextBox ID="txtBody" runat="server" TextMode = "MultiLine" Height = "150" Width = "200"></asp:TextBox>
        </td>
    </tr>
    <tr>
        <td>
            &nbsp;
        </td>
    </tr>
    <tr>
        <td>
            File Attachment:
        </td>
        <td>
            <asp:FileUpload ID="fuAttachment" runat="server" />
        </td>
    </tr>
    <tr>
        <td>
            &nbsp;
        </td>
    </tr>
    <tr>
        <td>
            Gmail Email:
        </td>
        <td>
            <asp:TextBox ID="txtEmail" runat="server"></asp:TextBox>
        </td>
    </tr>
    <tr>
        <td>
            &nbsp;
        </td>
    </tr>
    <tr>
        <td>
            Gmail Password:
        </td>
        <td>
            <asp:TextBox ID="txtPassword" runat="server" TextMode = "Password"></asp:TextBox>
        </td>
    </tr>
    <tr>
        <td>
            &nbsp;
        </td>
    </tr>
    <tr>
        <td>
        </td>
        <td>
            <asp:Button Text="Send" OnClick="SendEmail" runat="server" />
        </td>
    </tr>
</table>
(ii).aspx.cs page
===================
using (MailMessage mm = new MailMessage(txtEmail.Text, txtTo.Text))
        {
            mm.Subject = txtSubject.Text;
            mm.Body = txtBody.Text;
            if (fuAttachment.HasFile)
            {
                string FileName = Path.GetFileName(fuAttachment.PostedFile.FileName);
                mm.Attachments.Add(new Attachment(fuAttachment.PostedFile.InputStream, FileName));
            }
            mm.IsBodyHtml = false;
            SmtpClient smtp = new SmtpClient();
            smtp.Host = "smtp.gmail.com";
            smtp.EnableSsl = true;
            NetworkCredential NetworkCred = new NetworkCredential(txtEmail.Text, txtPassword.Text);
            smtp.UseDefaultCredentials = true;
            smtp.Credentials = NetworkCred;
            smtp.Port = 587;
            smtp.Send(mm);
            ClientScript.RegisterStartupScript(GetType(), "alert", "alert('Email sent.');", true);
        }

Note: by using this we can face one issue "

The server response was: 5.5.1 Authentication Required in ASP.Net Application." issue this is because of  gmail security. to over come this and from email should not have any 2 step verification code.

https://www.google.com/settings/security/lesssecureapps

Once you visit the link you need to modify the Less Secure Apps setting and Turn On access to Less Secure Apps as shown below.