I send mail excel file to my mail

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C#3.5

For that code as follows

MailMessage mail = new MailMessage();
SmtpClient SmtpServer = new SmtpClient("smtp.gmail.com");
mail.From = new MailAddress("rajesh@gmail.com");
mail.To.Add("sathish@gmail.com");
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("rajesh@gmail.com", "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.

What I have tried:

i send mail excel file to my 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);