Tracking e-mail opens is directly impacted by the recipient’s e-mail software, because tracking an open can only occur when an e-mail’s HTML is loaded. Not all e-mail software loads the full HTML automatically, so automating based on an e-mail open is not super reliable. Despite this instability in the automation confidence, automating based on e-mail opens can still be leveraged by another legacy merge field hack.
Infusionsoft Cookbook – Chapter 10.3 – Triggering Automation From An E-mail Open
