Mine (Gmail) is not, but many (Outlook, OSX Mail) allow this by default. The URL of the tracking image contains a number or hash that is used to uniquely identify the person opening up the email.
Outlook hasn't done this by default for at least two if not three versions now. However if you read a heavily stylized with images email you'll almost certainly tell it to download images (just as one does in gmail), giving them that feedback that they want.
Good to know, it's been several years since I used Outlook.