Symbol in email subject box

Since installing v9, I have started getting a symbol in the subject box for some of the emails in my inbox. It is at the right hand end of the subject box. It is a circle with small dashes at top, bottom, left and right. A small portion of a screen shot shows the symbol. What does it mean? Is it something that can be turned off?

Simon

This shows that there is a tracking pixel in the message.

You can customize the columns and remove that one, if you wish.

Thanks Gary for the quick response.