Disable DNS prefetching

server.go

@@ -305,6 +305,8 @@ func New(e *echo.Echo, options *Options) (*Server, error) {
 			// `style-src 'unsafe-inline'` is required for e-mails with
 			// embedded stylesheets
 			ectx.Response().Header().Set("Content-Security-Policy", "default-src 'self'; style-src 'self' 'unsafe-inline'")
+			// DNS prefetching has privacy implications
+			ectx.Response().Header().Set("X-DNS-Prefetch-Control", "off")
 			return next(ectx)
 		}
 	})