Why do some contacts who have opened the email not show up in the email‘s opens report?

  An open is recorded when an invisible 1px by 1px tracking image in your email displays on your contact's screen. It is also recorded when the recipient clicks on any of the URLs in your campaign that include link tracking. Benchmark Email cannot record opens when: ...the recipient does not enable images to display in an HTML version of your email. ...the recipient's firewall or spam filter removes images