Email Browsing Not Secure for All Messages

 The address bar changes from Secure to Not Secure right after loading mail and clicking on a message.  It does not happen to all messages,  But as soon as I click on a mail message "not secure" appears in the address bar .  As another user explained, "It doesn't matter if the email is of personal nature or from places that I have signed up for newsletters".   It  does not appear that there is a fix.  It happens in Microsoft Edge and Google Chrome.  The problem seems to lie in the Xfinity Email application.    "In fact, even the Xfinity newsletters have this response." 

Please advise on whether or not email messages are truly safe and that the Xfinity Email application can be trusted/secure.