Recently, I often receive mails with the warning as in the subject line above.

I've noticed that some old mails in my mailbox when opened now have this warning even they were received with no warning in the past. Also, some of the newer mails were received with warning, but the warning disappeared later.

The help given following the ? mark wasn't very informative.

Can you please give me more explanation on what's wrong with these mails?

Best regards & TIA