Check, if your mail client is created in this way ?
System.Net.Mail.SmtpClient mailClient = new SmtpClient("your.smtp.server", 25);
System.Net.NetworkCredential nc = new System.Net.NetworkCredential("YourLogin", "YourPassword");
mailClient.Credentials = nc;
And also check address of your SMTP server, login and password, if they are all correct. Or there may be also problems at server side.