Using SmtpClient, and getting "the target machine actively refused it"

I am trying to use System.Net.Mail for an application to send an email, but get this exception:

System.Net.Mail.SmtpException: Failure sending mail. ---> System.Net.WebException: Unable to connect to the remote server ---> System.Net.Sockets.SocketException: No connection could be made because the target machine actively refused it 198.238.39.170:25

The code I am using is:

string mailserver = "Scanmail.ofm.wa.lcl";
MailMessage msg = new MailMessage("albert@einstein.net", "snark@snarky.com", "Very subjective", "A message body!");
SmtpClient client = new SmtpClient(mailserver);
client.Send(msg);

Obviously the email addresses above are fictional, but in the actual code it uses real email addresses in the system. The mail server address is accurate.

I am tempted to think that I need to put some kind of security credentials in there, but not sure where - although @Andre_Calil's advice suggests this is not the problem, and that possibly the mail server is configured to prevent my development machine from connecting. So how is this to be overcome?