ReadNotify puts a read receipt request in the header:

Notice-Requested-Upon-Delivery-To:[email protected]

Most email clients and webmail clients will prompt you before sending a read receipt, except some old versions of Outlook.

They also put some images and sounds in the email. Again, most clients are configured to block external content unless you turn it on:

<img
src=http://www.o5htrp62cnbl18.ReadNotify.com/nocache/o5htrp62cnbl18/footer0.gif>

<BgSound src=https://tssls.o5j6tpo7cnbl7v.ReadNotify.com/nocache/o5j6tp07cnbl7v/rspr47.wav>

<td background=http://0320.185.27565/nocache/o5j6tpo7cnbl7v/rspr7q.gif>