Do companies know you've opened an email?

elektra

The following text was at the bottom of one of my emails, unusual for a company to be upfront with this info.

This email uses clear gifs (web beacons), which tell us whether you open this email. The information is used to enable more accurate reporting and ensure we are sending you relevant content. You can turn off web beacons by not "enabling images" in an email. Instructions on how to do this automatically should be in the help section or help file of your browser or program.