у меня была такая же проблема
Вы должны установить clietn.port как 25 и указать свой логин и пароль в client.Credentials = новый NetworkCredential (логин, пароль)
когда я это сделал, я могу отправлять почту без проблем
есть код
{
SmtpClient client = new SmtpClient("188.125.69.59");//you can put the ip adress or the dns of smtp server (smtp.mail.yahoo.com as exemple)
// Specify the e-mail sender.
// Create a mailing address that includes a UTF8 character
// in the display name.
MailAddress from = new MailAddress("from@yahoo.fr");
// Set destinations for the e-mail message.
MailAddress to = new MailAddress("to@gmail.com");
// Specify the message content.
MailMessage message = new MailMessage(from, to);
message.Body = "This is a test e-mail message sent by an application. ";
// Include some non-ASCII characters in body and subject.
string someArrows = new string(new char[] {'\u2190', '\u2191', '\u2192', '\u2193'});
message.Body ="cc";
message.BodyEncoding = System.Text.Encoding.UTF8;
message.Subject = "test message 1";
message.SubjectEncoding = System.Text.Encoding.UTF8;
string userState = "test message1";
MessageBox.Show("sending");
client.Port = 25;
// client.Timeout = 40000;
client.ServicePoint.MaxIdleTime = 1;
client.Credentials = new System.Net.NetworkCredential("from@yahoo.fr", "pass");
//client.SendAsync(message, userState);
client.Send(message);
MessageBox.Show("Sending message... press c to cancel mail. Press any other key to exit.");
string answer = Console.ReadLine();
// If the user canceled the send, and mail hasn't been sent yet,
// then cancel the pending operation.
// if (answer.StartsWith("c") && mailSent == false)
//{
// client.SendAsyncCancel();
//}
// Clean up.
message.Dispose();
MessageBox.Show("Goodbye.");
}