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:
We recommend placing at least one http:// link into your text-only emails to take advantage of the fact that link click-throughs are tracked as opens.
Note: This will only work for http:// links and not the secure https:// versions.