public class MailSender : IDisposable
    {
        SmtpClient SC;
        MailMessage Mess;
        public MailSender(string subj, string body, string F_Att)
        {
            SC = new SmtpClient("192.168.55.55", 25);
            SC.Credentials = new NetworkCredential("username", "password");
            Mess = new MailMessage("mail@mail.ru", "mail@mail.ru");
            Mess.Body = body;
            Mess.Subject = subj;
            Mess.Attachments.Add(new Attachment(F_Att));
        }
        public void Send()
        {
            SC.Send(Mess);
        }
        public void Dispose()
        {            
            GC.SuppressFinalize(this);
        }
    }

        private void Upl_USZN_NewInst()
        {
            DataTable DT = (DataTable)DM.Get_UplUsznFileList(100000, Convert.ToInt32(Program.SPID));
            string Dir = @"c:\DIR\";
            //DT.Rows:
            //100000 | file1.dbf
            //100001 | file2.dbf
            //100000 | file3.dbf
            //100001 | file4.dbf
            foreach (DataRow R in DT.Rows)
            {                
                    File.WriteAllBytes(Dir + R["FileUpload"].ToString(), Properties.Resources.empty);
                    //подготовка файла для отправки по почте
                    DM.SP_Upload_Uszn(100000, Convert.ToInt32(Program.SPID), Dir, R["FileUpload"].ToString());
                    if (R["SubDivisionID"].ToString() == "100000")
                    {
                        using (MailSender MS = new MailSender("subj0", "", Dir + R["FileUpload"].ToString()))
                        {
                            MS.Send();
                        }
                    }
                    if (R["SubDivisionID"].ToString() == "100001")
                    {
                        using (MailSender MS = new MailSender("subj1", "", Dir + R["FileUpload"].ToString()))
                        {
                            MS.Send();
                        }
                    }
            }
            F_Refresh();
            MessageBox.Show("Файлы выгружены");
            return;
        }

SC.Dispose() и
Mess.Dispose()

using (MailSender MS = new MailSender(R["SubDivisionID"].ToString() == "100000" ? "subj0" : "subj1", "", Dir + R["FileUpload"].ToString()))
{
    MS.Send();
}