I found some topics about this thing, but I tried many ways to resolve this, but not working yet :( I received the mail but went to Junk folder.
This is my code:
MailMessage mail = new MailMessage();
SmtpClient smtpServer = new SmtpClient("<WORKING SMTP>");
smtpServer.Credentials = new System.Net.NetworkCredential("<WORKING USER NAME>", "<WORKING PASS>");
smtpServer.Port = 26; // Gmail works on this port
mail.From = new MailAddress("<SENDER MAIL>");
mail.To.Add(addressee);
mail.Subject = subject;
mail.Body = body;
mail.IsBodyHtml = true;
mail.BodyEncoding = System.Text.Encoding.GetEncoding("utf-8");
ContentType mimeType = new System.Net.Mime.ContentType("text/html");
AlternateView alternate = AlternateView.CreateAlternateViewFromString(body, mimeType);
mail.AlternateViews.Add(alternate);
smtpServer.Send(mail);