Both System.Net.Mail.MailMessage & System.Net.Mail.SmtpClient are IDisposable classes. You can try the following,
using (System.Net.Mail.SmtpClient client = new System.Net.Mail.SmtpClient("ares"))
{
using (System.Net.Mail.MailMessage msg = new System.Net.Mail.MailMessage())
{
msg.From = new MailAddress("system@lol.dk");
msg.To.Add(new MailAddress("lmy@lol.dk"));
msg.Subject = "IBM PUDO";
msg.Body = sentFiles.Count() + " attached file(s) has been sent to the customer(s) in question ";
msg.IsBodyHtml = true;
foreach (string file in sentFiles)
{
Attachment attachment = new Attachment(file);
msg.Attachments.Add(attachment);
}
client.Send(msg);
}
}