This article gives no new information, it is stupid.
Now i will speculate:
As long as he does not know his customers passwords he can't retroactively view the customers mails, once the mails have been encrypted and the plain-text thrown away the stuff is unreachable.
So the US gov probably wanted him to save his customers passwords when they logged in.
Right and the important point being that this isn't just about sniffing his network, it is the most invasive surveillance possible that would make his whole business a lie. The point of the article was to go through the logic to show that this would have been the only possibility for what they requested him to do.
Now i will speculate:
As long as he does not know his customers passwords he can't retroactively view the customers mails, once the mails have been encrypted and the plain-text thrown away the stuff is unreachable.
So the US gov probably wanted him to save his customers passwords when they logged in.