**SPAM** in subject column

Over the last few weeks an annoying thing has happened to my inbox. The Following appears infront of an othe rwording in the subject column - **SPAM**. Sometime the email concerned is spam, often it's not. I don't know how **SPAM** has got there nor how to remove it. I use PC Tools Internet Security and Malwarebytes but can't fnd anything in these apps that could be the culprit. Is it something in Windows Mail itself or could it be comeing from my ISP?