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/
Paraphrasing:
One of these studies revealed that the average length of emails sent was 41-50 characters. This study analysed 9 million emails which were sent in February 2015. This seems to suggest that my calculated average length of emails is consistent with, or maybe slightly shorter than, the average length of mass-marketing emails.
The grammar in this sentence is wrong, particularly: "suggest that at marketing mail at that time...". But looking at the rest of the article one can guess at the author's intention. Hope this helps!
