http://developers.de/blogs/damir_dobric/archive/2013/05/23/smtpclient-error.aspxWhen working with SMTP client, you might get following error: The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.7.1 Client was not authenticated There are lot of answers out there related thisenCommunityServer 2008 SP1 (Build: 30619.63)