I guess it's a poorly composed sentence, as I've read it 10 times and still can't get the main idea.
One of these studies, which analysed 9 million emails, let drop that the average length of 9 million emails sent in February 2015 was 41-50 characters, which seems to suggest that at marketing mail at that moment in time matches my average, or maybe slightly shorter. http://www.oblomovka.com/wp/2016/02/17/are-email-subject-lines-getting-longer/