Seems to work just like spam, with a lazy html tag.

This won't work if you are offline, or if you use a proper mail client that blocks external http requests (like Pegasus [:)], or just about everything now).

It looks like it inserts itself as an outgoing mail proxy between the client and the SMTP server and inserts the tracking tag and advertising spam into your mail.

I wouldn't touch this with YOUR bargepole [:D].

What is wrong with 'requesting a read reciept'?

Here is another thread about MSGTAG.

http://groups.google.co.nz/group/alt.comp.freeware/browse_thread/thread/3741d50cb15154b2/aa0c851059bf2501?hl=en&lnk=st&q=MSGTAG#aa0c851059bf2501 

Does anyone use MSGTAG successfully with Pegasus? It routes all your emails through it's server (I think) and tells you when someone has viewed the email that you sent them. I got an error message yesterday when I tried it, and their website is down, so I might have another look when I get home tonight.