MailAddress from = new MailAddress("email", "name");
MailAddress to = new MailAddress("email");
MailMessage m = new MailMessage(from, to);
m.Body = $"text";
SmtpClient smtp = new SmtpClient("server", 25);
smtp.UseDefaultCredentials = false;
smtp.Credentials = new NetworkCredential("email", "password");
smtp.EnableSsl = false;
smtp.Send(m);
проверял через smtp тестер