If you are seeing that your connection is not secure in the address bar of the browser, this is usually down to images in either emails that you have opened or images in your templates that are from an outside web address that is not secure, in terms of the emails you receive you can make sure you keep the show Images box un-ticked. For images in your templates instead of copying them in from an outside source you need to upload them using the image uploader that is available in the editor, this will then upload the image to HQ which will make it secure.