Interesting, but is it really true 'normal' open rates are also only 13%? Or is this how it's measured, e.g. only including people who click 'show images', and thus load the tracking pixels?
Typically the open rate is the combination of unique users who download images (along with the tracking pixel embedded) and unique users who click on tracked links from within the email.
Open rate does not include the people who view the email without images, unless those people click a link in the email.
A standard open rate (across email clients) is between 15-30%. However it's possible to consistently hit a 50-60% open rate, if you know what you are doing, basically meaning using correct techniques and content / offers that are really relevant to your recipients. BoringCo sending emails titled "March newsletter" will get 5-10% typically.
