C# ASP.NET Send Email via TLS
I was almost using the same technology as you did, however I was using my app to connect an Exchange Server via Office 365 platform on WinForms. I too had the same issue as you did, but was able to accomplish by using code which has slight modification of what others have given above.
SmtpClient client = new SmtpClient(exchangeServer, 587);client.Credentials = new System.Net.NetworkCredential(username, password);client.EnableSsl = true;client.Send(msg);I had to use the Port 587, which is of course the default port over TSL and the did the authentication.