Mail from one of my clients is being filtered at Postini and they asked me to look into this. Not that there is anything that can be done, of course. Even before they were bought out by Google, they were the poster child for a spam filtering company that believed they could do no wrong. It was difficult, if not impossible to get a straight answer from Postini about filtering, and the only statement they would ever make in regards to blocking problems was ‘have the recipient whitelist your mail.’
It is not just that Postini will not talk with people who are blocked, they will not talk to their own customers, either. Many years ago, I was dealing with another Postini issue for a customer. This customer was a Postini customer and was sending mail to themselves to test their new ESP. Postini was blocking the mail and the customer wanted me to find out why. After a couple days of digging I did actually find a really-o truly-o human at Postini.  He explained to me that a single line of text, followed by an unsubscribe link was spam, always spam and nothing but spam. He also explained that the only way for that mail to be let through, was for my customer to turn off his Postini filters.
Fast forward 4 years and I once again have a customer blocked by Postini. Usually, I tell customers there is nothing to be done for Postini blocks and that no one can find any information about them, but this customer is insistent. This particular customer has extremely clean mailing practices, sends highly relevant and wanted mail and consistently gets 95+% inbox delivery. They are not spammers, not even a little bit. Because I know this customer is so clean, I poked around a little to find some information about them. They do use the ReturnPath Mailbox Monitor so I have a copy of the headers Postini is adding. I also discovered that Postini is now providing a decoder service for their headers at https://www.postini.com/support/header_analyzer.php
The response you get back from pasting in a header is not that useful if you have found any of the numerous explanations of Postini headers, but it does show some willing. Note, there is no way to ask a question or provide feedback to Postini on the listing.
There is not much that can be done to deal with Postini filtering your email. The best you can do is have your recipients whitelist you.
 I believe I am the only person on the delivery end that has ever been able to actually talk to a live human at Postini, and I think that is only because I called them from the same area code they are in and some engineer decided to return the message I left on their corporate voicemail.