Code:
public void Send(string from, System.Collections.Hashtable BccAddresses, string subject, string body, string attachmentFilePath)
        {
            try
            {

                MailMessage mail = new MailMessage();
                mail.Body = body;
                mail.Subject = subject;
                mail.From = new MailAddress(from);

                foreach (System.Collections.DictionaryEntry de in BccAddresses)
                {
                    mail.Bcc.Add(de.Value.ToString());
                }
                if (attachmentFilePath.Length > 0)
                {
                    Attachment attach = new Attachment(attachmentFilePath);
                    mail.Attachments.Add(attach);
                }
                SmtpClient smtp = new SmtpClient();
                smtp.Host = _smtpserver;
                smtp.DeliveryMethod = SmtpDeliveryMethod.PickupDirectoryFromIis;
                if (_usecredentials.Equals("1"))
                {
                    smtp.UseDefaultCredentials = false;
                    smtp.Credentials = System.Net.CredentialCache.DefaultNetworkCredentials;

                    System.Net.NetworkCredential networkCredential = new System.Net.NetworkCredential();
                    networkCredential.Domain = _domain;
                    networkCredential.UserName = _smtpusername;
                    networkCredential.Password = _smtppassword;
                    smtp.Credentials = networkCredential;
                }
                smtp.Send(mail);
            }
            catch (Exception ex)
            {
#if DEBUG
                System.Diagnostics.Debug.WriteLine("Mail not sent! - " + ex.Message);
#endif
                throw ex;

            }
        }