邮件是从wpf应用程序发送的，但不能从某些电脑发送

Question

邮件是从wpf应用程序发送的，但无法从某些电脑发送 当我在我的电脑上运行安装程序时，邮件发送正确，但当我从另一个电子邮件重复此过程时，邮件不会被发送，邮件是“邮件未发送”;

                SmtpClient smtp = new SmtpClient();
                smtp.Host = "mail.test.com";
                smtp.UseDefaultCredentials = false;
                smtp.Credentials = new System.Net.NetworkCredential("test@test.com", "Test");
                string mailTo = emailId;
                string mailFrom = "test@test.com";
                string mailSub = subject;
                string mailMessage = strMessage;
                MailMessage message = new MailMessage(mailFrom, mailTo, mailSub, mailMessage);    

                message.BodyEncoding = System.Text.Encoding.UTF8;
                message.IsBodyHtml = true;

                try
                {
                    smtp.Send(message);
                    return "Send";
                }
                catch (Exception ex)
                {
                    MessageBox.Show(ex.Message.ToString());
                    return "Failure";
                    //return ex.ToString();
                }