It's the way most current web browsers are designed. Whenever you use any provider's webmail to open an email that references a non-secure (loaded with "http" instead of "https") image, script, or other item, the browser warns you. In webmail, this probably happens most often if you have the setting for "Allow pre-loading of externally linked images" (gear icon / Settings / Security / Mail) checked. If you don't have that option checked, you're probably getting the browser warning when you open an email that has insecure content and you click the "Show images" button.
Sadly, many of the emails Comcast sends have insecure image links and will trigger the browser's warning. Two examples are "http://log.dmtry.com/" and "http://xfinity.comcast.net/favicon.ico". To a browser both of these are "insecure" because they use "http" instead of "https".
A secure web page that loads insecure content is said to be using "mixed content", and may be subject to a "man-in-the-middle" attack. Comcast seems to think this is not a problem, but Internet security experts think otherwise. See https://www.google.com/search?q=mixed+content+pages+MITM.
Since comcast email is on a secure address as I stated in my post. No email should register as not secure while still displaying https. No other email I use does this. Only comcast!
I am experiencing the same problem. Initially, some of the responses on Google suggested that this could be a problem of Google Chrome Browser but even with other browsers such as Firefox, I get the same alert as soon as I log into my email account.
I wonder if Comcast is reviewing this and offering in any solutions?
I've been back and forth with Comcast for 2 weeks now about my email not being secure as others here have stated. Also when I send an email to myself from my cell phone so I can pick it up on my laptop, it's going straight to spam. Security Department said they have to escalate my request and get back to me. If they call and I'm not available, they only give me 2 more callbacks before they just close the ticket. Not very helpful!
I was successful in speaking with Comcast (level 1 & level 2) tech.s who eventually had instant messaging with their level 3 tech. on this problem.
As per them the change from "Secure" (with Padlock) to "Not Secure" (with an i either within a circle or in a shaded triangle) occurs on the URL since certain portion of the content - such as images etc.: in the email(s) or ad's or on the loaded page, in our said mail box has a lower level of encryption than required for "secure" certification.
However, as per them, once we have logged into our secure email ID, all content of emails sent or being sent - remains secure.
Of course it is important to be cautious about any spam or phishing mails in our mail box.
Hopefully this helps.