Something about their analysis just doesn't sound right.

"I dug deep into our database and pulled every delivery to Gmail for the past year and half. That amounts to 12.5 billion emails".

Then he says

"As you might suspect, Gmail opens do follow these hourly, weekly, and seasonal trends, but on the whole they remain fairly steady. With that settled, I trimmed my data to a lean 6 weeks around the introduction of tabs. That’s about 1.5 billion emails, which is plenty of records for a good analysis."

So, 1.5 years = 12.5B Gmail emails

But last 6 weeks = 1.5B Gmail emails

That doesn't sound right.

Unless MailChimp is on some type of unbelievable (true) exponential growth rate in the past few months.

Edit: typo