Have you clicked on the icon where it says it is not secure. It will inform you what is not secure and why you are getting the message. It is usually because of an image or graphic in the email.
Thank you gg30340. You are correct.
Can we just ignore this then? I lot seem to come from well known sites.
You are being given information about the contents or part of the content of the email that has been sent to you. It is entirely your choice what you do with that information.
Nobody can give you a 100% "just ignore it" because without checking what the warning is about it is impossible to say it that it is safe to ignore it. You should check and then decide what you want to do with the email.
I've noticed the same thing in Safari. Padlock is there when I go to BT Email. If I click on a 'regular' email (e.g. plain text) the padlock stays. Click on an email from, say, Waterstones and the padlock vanishes. However, if I forward these emails to my gmail account the padlock always remains on with gmail when opening the email.
That said, running Wireshark while opening these emails does reveal they contain images that are sent via http and not https. Quite why this doesn't seem to affect gmail is beyond me though.