What I got from reading the excerpt of Judge Koh's response in the article was that she was maintaining a vertical separation between the business / activities of sending and receiving mail and other business activities. From my following, I'd imagine that filters and priority inbox and other details would be related to the business of sending and receiving mail.

If Google can win the argument that the way they're using their interception and reading of messages in serving personalized ads is a part of their everyday business activities, it seems to completely undermine the purpose of the law. Google, with G+, is trying to itself be a single unified product with all of these features. Social networking is a feature, videos are a feature, websites, blogs, instant messaging and email and targeted advertising. If Google can say that displaying targeted ads is a part of their normal business activities, then what activity does the law against wiretapping actually protect against?

If Google can read your email for the purpose of enabling a tangential feature of displaying targeted advertising to you, how does that same logic not apply to me if I claim my normal business activities are delivering your mail while opening it, reading it, and selling useful information I find inside of it? I could say it was essential to my business, because I actually deliver for free and just pay my bills with the info I sell.