MailAddress from = new MailAddress("somemail@gmail.com", "Y");
MailAddress to = new MailAddress("some@mail.ru");
MailMessage m = new MailMessage(from, to);
m.Subject = "Тест";
m.Body = "<h2>Письмо</h2>";
m.IsBodyHtml = true;
SmtpClient smtp = new SmtpClient("localhost");
smtp.EnableSsl = true;
try
{
smtp.Send(m);
}
catch (Exception ex)
{
Console.WriteLine(ex.ToString());
}
Console.Read();

SmtpClient client = new SmtpClient();
client.Port = 587;
client.Host = "smtp.gmail.com";
client.EnableSsl = true;
client.Timeout = 10000;
client.DeliveryMethod = SmtpDeliveryMethod.Network;
client.UseDefaultCredentials = false;
client.Credentials = new System.Net.NetworkCredential("user@gmail.com","password");

MailMessage mm = new MailMessage("donotreply@domain.com", "sendtomyemail@domain.co.uk", "test", "test");
mm.BodyEncoding = UTF8Encoding.UTF8;
mm.DeliveryNotificationOptions = DeliveryNotificationOptions.OnFailure;

client.Send(mm);