Can't Send Emails From my Website - asp.net

Question

Here I am in a strange situation. When I send the email from localhost it is working fine but when I upload the page to the server and try to send email, I get the following error
The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.5.1 Authentication Required
I'm displaying the error message in a label for testing purpose and the try is also misplaced I know, I will set it later.
the code I am using is
if (Page.IsValid)
{
try
{
StringBuilder message = new StringBuilder();
message.Append("Hello My Name is ");
message.Append(txtName.Text);
message.AppendLine();
message.AppendLine("My Contact Number " + txtContactNumber.Text.ToString());
message.AppendLine();
message.AppendLine();
message.AppendLine("My Email Id Is " + txtFromEmailAddress.Text.ToString());
message.AppendLine();
message.Append(txtEmailBody.Text);
MailMessage mailMessage = new MailMessage("xxx#gmail.com", "yyy#gmail.com");
mailMessage.Subject = "New Client Query";
mailMessage.Body = message.ToString();
SmtpClient smtpClient = new SmtpClient("smtp.gmail.com", 25);
//smtpClient.UseDefaultCredentials = true;
smtpClient.Credentials = new System.Net.NetworkCredential()
{
UserName = "xxx#gmail.com",
Password = "password"
};
smtpClient.EnableSsl = true;
smtpClient.Send(mailMessage);
txtContactNumber.Text = "";
txtFromEmailAddress.Text = "";
txtName.Text = "";
txtEmailBody.Text = "";
lblEmailStatus.Text = "Email Sent Successfully.";
lblEmailStatus.ForeColor = System.Drawing.Color.Yellow;
}
catch(Exception ex)
{
lblEmailStatus.Text = ex.Message + " <br> " + ex.Source;
}
}
else
{
lblEmailStatus.Text = "Error! Email Not Sent ";
lblEmailStatus.ForeColor = System.Drawing.Color.Yellow;
}
I have googled for a couple of hours and checked links at this site as well but I still cant figure it out.
Now I request you guys here if any one have any solutions / hint.

Answer 1

Try this
public string SendEmailTest(String EmailMessage, String FromMail, String MailPassword, String MailServer, String To, String CC, String BCC, String DisplayName, String Subject, String Attachment)
{
try
{
SmtpClient smtpClient = new SmtpClient();
MailMessage message = new MailMessage();
MailAddress fromAddress;
fromAddress = new MailAddress(FromMail);
smtpClient.Host = MailServer;
smtpClient.Port = 25;
System.Net.NetworkCredential SMTPUserInfo = new System.Net.NetworkCredential(FromMail, MailPassword);
smtpClient.UseDefaultCredentials = false;
smtpClient.Credentials = SMTPUserInfo;
message.From = fromAddress;
message.To.Add(new MailAddress(To, DisplayName));
if (CC != "")
message.CC.Add(new MailAddress(CC, DisplayName));
if (BCC != "")
message.Bcc.Add(new MailAddress(BCC, DisplayName));
message.Subject = Subject;
message.IsBodyHtml = true;
message.Body = EmailMessage;
if (Attachment != "")
message.Attachments.Add(new Attachment(Attachment));
message.Priority = MailPriority.High;
smtpClient.Send(message);
return "SendEmail";
}
catch (Exception ex)
{
return "Email :" + ex;
}
}

Answer 2

i got the reason finally .
the email i was sending emails from was kind of hacked some days before and for the security reasons google team had kind of marked my email as unsecure . i changed the emails address and it is working fine thanks you all.

Answer 3

Based on the Google Gmail Documentation it would appear that the Port should be 587 not 25. I found a few other questions that seem to be related here and here.
SmtpClient smtpClient = new SmtpClient("smtp.gmail.com", 587);
client.UseDefaultCredentials = false;