i am sending excel file to my mail

My code as follows

MailMessage mail = new MailMessage();

SmtpClient SmtpServer = new SmtpClient("smtp.gmail.com");

mail.From = new MailAddress("[email protected]");

mail.To.Add("[email protected]");

mail.Subject = "Test Mail - 1";

mail.Body = "mail with attachment";

System.Net.Mail.Attachment attachment;

attachment = new System.Net.Mail.Attachment(@"C:\Users\551386\Desktop\DataDump\Excel.xls");

mail.Attachments.Add(attachment);

SmtpServer.Port = 587;

SmtpServer.Credentials = new System.Net.NetworkCredential("raj@gmailcom","12345");

SmtpServer.EnableSsl = true;

SmtpServer.Send(mail);

when i  execute the above code shows error as follows

  Failure sending mail

the above error shows in below line as follows

  SmtpServer.Send(mail);

please help me what is the mistake in my above code.