This could be a .Net bug, described at: http://blogs.msdn.com/b/webdav_101/archive/2008/06/02/system-net-mail-with-ssl-to-authenticate-against-port-465.aspx

TL;DR: SSL + port 465 (implicit SSL) is not supported in .Net. You can only use SSL + port 25 (explicit SSL)

See http://stackoverflow.com/questions/35228313/nlog-mail-target-timeouts/35235850#35235850