Why are email senders placing tracking images at the end of the content?

Question

All the email providers are placing zero pixel images at the end of the content for tracking opens. Is there any particular reason? For example:

<img src = "http://tracksamle.com/ssss/track.gif" width="0" height="0">

Answer 1

Someone may be able to explain to larger detail, but I'm guessing it is forcing the email client to fetch the image from said address (which I would guess is unique such that it can be cross linked to the intended receiver of the email).

By then, on server, you can determine if the email has indeed been opened, and the IP-address of the opening party.

How this interferes with spam filters and other in-transit email scanners, I do not know though.

Edit: As @bartdude also mentions in comments, positioning at end imposes least inconvenience on load times for the reader, as by then rest of the email should already be loaded, albeit this depends on implementation of loading process.

Answer 2

Adding to the answer above, note that if someone opens an email and has the option to "load images" and they don't click the link to load them, it will not count as an open. This is true also for text only emails. So if someone is tracking opens, they will never be accurate because of this.