ti faccio un esempio con gmail:

string username = "tuonick@gmail.com";
string pwd = @"tuapass";

System.Net.Mail.MailMessage message = new System.Net.Mail.MailMessage(new MailAddress("mail address"), new MailAddress("mail address"));
message.Body = "body";
message.Subject = "subject";

System.Net.Mail.SmtpClient smtp = new System.Net.Mail.SmtpClient("smtp.gmail.com", 587);
smtp.DeliveryMethod = System.Net.Mail.SmtpDeliveryMethod.Network;
System.Net.NetworkCredential oCredential = new System.Net.NetworkCredential(username, pwd);
smtp.UseDefaultCredentials = false;
smtp.Credentials = oCredential;
smtp.EnableSsl = true;
smtp.Send(message);