Here you select the events for which you want notifications, by private message and/or by email. These are all self-explanatory except for the last, reads an email that I sent. If this is checked, outgoing emails will have an image reference added to the end of the email, that looks like this:

 

 

This image actually lives on the QDL server, and when it is accessed by the email client, QDL can tell that your email was opened, and you get PM'd. But the absence of such a PM doesn't mean that the email was not read. If the recipient has images turned off, the image won't display, and QDL never gets word back. So if you get a PM, the message was definitely read. If you don't, maybe it was read and maybe it wasn't.