SMTP E-mail

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C#

I have created a small program that generates a PDF and e-mails it to the intended user, using the "localhost" as the smtp server. This works on my local machine. However when I move the program to a server the e-mail are never recieved. If I alter the program and use the ip of the mail host it will work. The problem is that the e-mail will be moving to an outside source. So there will no longer be and internal e-mail server. Here is the code simple just in case anyone has any questions. XXX are where the IP address would be.

Anyone have any ideas

private void SendMailWithAttachment(string strFrom, string strTo, string strSubject, string strBody, Attachment attOrder, Attachment attPacking)
        {
            try
            {
                MailMessage mail = new MailMessage(strFrom, strTo, strSubject, strBody);
                mail.Attachments.Add(attOrder);
                mail.Attachments.Add(attPacking);
                SmtpClient smtp = new SmtpClient("xxx.xxx.xxx.xxx");
                smtp.Send(mail);
                mail.Dispose();
            }
            catch(Exception ex)
            {
                string strError = ex.Message;
                //txtBigText.Visible = true;
                //txtBigText.Text = "Mail w/ Attachment Not Sent";
            }
        }