Unable to send email programatically

For some reason I am unable to send email programatically from my account.

here is my code (there are valid values for the variables):

MailMessage mm = new MailMessage();
mm.From = new MailAddress(from);
mm.To.Add(to);
mm.Subject = subject;
mm.Body = body;
SmtpClient client = new SmtpClient("localhost");
client.Send(mm);

This seemed to work for a day then stopped for some reason. Any ideas?