I think there are two main reasons that explain why Google has acquired Postini:

- Archiving as a way to get large clients on Google Apps when they face legal requirements (SOX...). I have already seen many requests in this field.

- A set of clients using Exchange or IBM Lotus Notes to whom it will be easier to propose a move to Google Apps.

I don't think Google will acquire an email company which has an offering for Intranet. Two reasons :
- Outside of Notes and Exchange, no vendor has a significant market share.

- This will go against the main strategy of Google, offering SaaS solutions which run on its ultra efficent infrastructures.

Of course, I may be wrong! Time will tell.