[Solved] Email attachment is corrupted (Zero bytes) when sending to multiple Addresses in ASP.Net

Use below modified code.

VB.Net

Private conn As String = ConfigurationManager.ConnectionStrings("***").ConnectionString
Protected Sub EmailfromTable()
    Using sqlCon As New SqlConnection(conn)
        Using cmd As New SqlCommand()
            cmd.CommandText = "Select Username, Email from users"
            cmd.Connection = sqlCon
            sqlCon.Open()
            Dim da As New SqlDataAdapter(cmd)
            Dim dt As New DataTable()
            da.Fill(dt)
            sqlCon.Close()
            Dim subject As String = ESubject.Text.Trim
            Dim body As String = "Hello {0}, <br /><br />" & EBody.Text.Trim
            Dim fileName As String = ""
            Dim bytes As Byte() = New Byte() {}
            If fuAttachment.HasFile Then
                fileName = Path.GetFileName(fuAttachment.PostedFile.FileName)
                Dim binaryReader = New BinaryReader(fuAttachment.PostedFile.InputStream)
                bytes = binaryReader.ReadBytes(fuAttachment.PostedFile.ContentLength)
            End If
            'Using Parallel Multi-Threading send multiple bulk email.
            Parallel.ForEach(dt.AsEnumerable(),
                        Function(row)
                            Return SendEmail(row("Email").ToString(), subject, String.Format(body, row("UserName")), bytes, fileName)
                        End Function)
        End Using
    End Using
    ClientScript.RegisterStartupScript(Me.GetType, "alert", "alert('Email sent.');", True)
End Sub

Private Function SendEmail(recipient As String, subject As String, body As String, bytes As Byte(), fileName As String) As Boolean
    Dim mm As New MailMessage("*****", recipient)
    mm.Subject = subject
    mm.Body = body
    mm.IsBodyHtml = True
    If Not String.IsNullOrEmpty(fileName) Then
        mm.Attachments.Add(New Attachment(New MemoryStream(bytes), fileName))
    End If
    Dim smtp As New SmtpClient()
    smtp.Host = "*********"
    smtp.EnableSsl = True
    Dim NetworkCred As New NetworkCredential()
    NetworkCred.UserName = "*********"
    NetworkCred.Password = "******"
    smtp.UseDefaultCredentials = False
    smtp.Credentials = NetworkCred
    smtp.Port = 587
    smtp.Send(mm)
    Return True
End Function

Protected Sub SendBulkEmail(sender As Object, e As EventArgs) Handles Button1.Click
    EmailfromTable()
End Sub

C#

    private string conn = ConfigurationManager.ConnectionStrings("***").ConnectionString;
    protected void EmailfromTable()
    {
        using (SqlConnection sqlCon = new SqlConnection(conn))
        {
            using (SqlCommand cmd = new SqlCommand())
            {
                cmd.CommandText = "Select Username, Email from users";
                cmd.Connection = sqlCon;
                sqlCon.Open();
                SqlDataAdapter da = new SqlDataAdapter(cmd);
                DataTable dt = new DataTable();
                da.Fill(dt);
                sqlCon.Close();
                string subject = ESubject.Text.Trim;
                string body = "Hello {0}, <br /><br />" + EBody.Text.Trim;
                string fileName = "";
                byte[] bytes = new byte[] { };
                if (fuAttachment.HasFile)
                {
                    fileName = Path.GetFileName(fuAttachment.PostedFile.FileName);
                    var binaryReader = new BinaryReader(fuAttachment.PostedFile.InputStream);
                    bytes = binaryReader.ReadBytes(fuAttachment.PostedFile.ContentLength);
                }
                // Using Parallel Multi-Threading send multiple bulk email.
                Parallel.ForEach(dt.AsEnumerable(), row =>
                {
                    return SendEmail(row("Email").ToString(), subject, string.Format(body, row("UserName")), bytes, fileName);
                });
            }
        }
        ClientScript.RegisterStartupScript(this.GetType(), "alert", "alert('Email sent.');", true);
    }

    private bool SendEmail(string recipient, string subject, string body, byte[] bytes, string fileName)
    {
        MailMessage mm = new MailMessage("*****", recipient);
        mm.Subject = subject;
        mm.Body = body;
        mm.IsBodyHtml = true;
        if (!string.IsNullOrEmpty(fileName))
        {
            mm.Attachments.Add(new Attachment(new MemoryStream(bytes), fileName));
        }
        SmtpClient smtp = new SmtpClient();
        smtp.Host = "*********";
        smtp.EnableSsl = true;
        NetworkCredential NetworkCred = new NetworkCredential();
        NetworkCred.UserName = "*********";
        NetworkCred.Password = "******";
        smtp.UseDefaultCredentials = false;
        smtp.Credentials = NetworkCred;
        smtp.Port = 587;
        smtp.Send(mm);
        return true;
    }

    protected void SendBulkEmail(object sender, EventArgs e)
    {
        EmailfromTable();
    }