When you see a spammers email, he/she can, in some cases see if it has been read.
There are programs such as msgtag (http://email.about.com/cs/receiptswindows/gr/msgtag.htm) and readnotify (http://www.readnotify.com/) that allow anyone to see if an email has been opened.
Msgtag relies on the user to load an image from a server. Therefore, once the image gets loaded, the sender is notified that the email is read.
Readnotify, on the other hand, uses a different method and can be setup to be undetectable. Something that is quite disturbing. Therefore, denying that you read the message from your boss and/or co-worker when you actually did can leave you in a very uncomfortable position.
This explains why email providers turn off the image feature.
Some email programs can keep track if an email is read, but they ask the recipient first. A feature that can be useful. Free email services do not have this feature. There maybe some that do, but I have not found any yet. Unfortunately, an email can still be read without sending a receipt. Therefore, the request receipt on read function relies on the honor system, and the recipient’s email provider allowing it.
Therefore, when you hear that opening emails from unknown senders is dangerous, it is.
Information about email tracking
HP tracks emails and attachments