People who want to trash this study with the same old "Have they thought of $obvious_counterargument?" should first browse through their GitHub repo: https://github.com/the-markup/investigation-wheres-my-email

I did and I have gone through the mbox files and I cannot find anything that would explain the curious difference. That is not to say that there isn't a rational explanation, the probably is, but please give the study authors some credit. They aren't dumb.