I have a client that says he's reading all of our emails (we distribute a daily client newsletter) but Marketo is only showing 2 opens in the last several months. The client said he's reading them in the 'reading pane' of Outlook. So, what constitutes a true open in Marketo?
Marketo adds a transparent tracking pixel as an <img> inside of all non-operational emails. If the recipient's email client attempts to load the image, Marketo knows the email was viewed by the recipient. If the recipient has "auto download images" turned off, then the open will NOT be registered until the user opts to "download images".
This in fact not 100% correct. Marketo will report a click as an open in the email performance report, but it will not log an "email opened" activity. The related triggers will not fire, and an "email opened" filter will not retrieve that lead.