One legitimate way to do this is to only send textual information in the email asking the user to follow a link which you can then track. Something along the lines of:
//lots of good info to identify yourself/company
Thank you for your request...
Please follow this link: http://somewhere/ for tracking information
Unfortunately this leaves it in the users hands to acknowledge your request. This is considered to be the most respectful way to do it.
If your application is not public facing (i.e. an internal app for your company), then the img src method already posted is a good technique.