Well, what I see, you didn't provide any user name/password, if your SMTP server supports relay.
Check the code,
smtpClient = new SmtpClient();
System.Net.NetworkCredential credentials = null;
smtpClient.Port = int_port;
smtpClient.Host = str_server;
if (str_ssl == "SSL")
{
smtpClient.EnableSsl = true;
}
else
{
smtpClient.EnableSsl = false;
}
if (str_isauthenticate == "Yes")
{
smtpClient.UseDefaultCredentials = false;
credentials =
new System.Net.NetworkCredential(str_user, str_password);
smtpClient.Credentials = credentials;
}
else
{
smtpClient.UseDefaultCredentials = true;
credentials =
new System.Net.NetworkCredential();
smtpClient.Credentials = credentials;
}
Hope this helps.
cheers