hello friend i have checked your code and i cant get any error
i just specified from and to email address and it will send testing email to my account successfully.
SmtpClient client;
System.Net.NetworkCredential NTLMAuthentication;
string _sysEmail = "xyz@abc.com";
string _sysPass = "xyzabc";
string _sysSMTP = "yoursmtpserver";
NTLMAuthentication = new System.Net.NetworkCredential(_sysEmail, _sysPass);
client = new SmtpClient(_sysSMTP);
client.UseDefaultCredentials = false;
client.Credentials = NTLMAuthentication;
System.Net.Mail.MailMessage myMsg = new MailMessage();
myMsg.From = (MailAddress)(new MailAddress("xyz@abc.com"));
myMsg.To.Add("abc@xyz.com");
myMsg.Body = "hi testing mail";
client.Send(myMsg);
please check this with replacing your server information and email addresses.