Almost exactly two years ago, I wrote a post about using TLS with SMTP. See Using TLS with SmtpClient If you’d like to send transport layer encrypted email with you SharePoint farm, you can do so by installing a .NET update and modifying the registry. https://blogs.msdn.microsoft.com/rodneyviana/2016/06/28/the-unofficial-guide-for-sharepoint-2013-and-2010-working-with-tls-1-2-only/