Web Forms :: Send Email To External Email Id Using C#?

i am trying to send emails to internal as well external email id(like gmail,yahoo,rediff etc.) .but the problem is that i can send the email to internal email id and recieve it but when i try to send the mail to external email id iam not able to receive the email. the process runs fine cause no errors are occurring.

SmtpClient emailClient;
emailClient = new SmtpClient("80.0.0.120", 252);
MailMessage message = new MailMessage("a.b@abc.com", a.b@glass.com", "Report", "Dear All,");
message.IsBodyHtml = true;
message.Body ="Dear All,"+"<br/>"+ "Please find the attached file.";
System.Net.Mime.ContentType mimeType = new System.Net.Mime.ContentType();
message.Attachments.Add(new System.Net.Mail.Attachment("Report.xls", mimeType));
emailClient.Credentials = new System.Net.NetworkCredential("a.b", "pass123");
emailClient.UseDefaultCredentials = false;
emailClient.Send(message);
emailClient = null;
message.Dispose();