Laura Atkins has a lively discussion going on about Postini’s lack of response to deliverability/blocking questions.

We’ve been frustrated and bewildered by random Postini blocks ourselves (see here and here). We gave up on trying to contact Postini a while ago.

But just recently, we had a customer sending tests to their VERY large client, and consistently getting blocked by their Postini filters.

We eventually got (real, live) people from Postini on the phone.

It went back and forth for a few days via phone and email. At first, it was the typical Postini situation. They said, “nothing we can do, it’s probably the sender.” So the IT folks at the client said, “it’s not our problem, it’s MailChimp. They’re known spammers” (Sigh. They always assume this, so they can get back to whatever they’re doing).

Then Dan, co-founder here at MailChimp, finally got sick and tired of the blame game and suggested there was a bug in Postini. He did this after thoroughly analyzing the bounce headers with our lead engineer. They discovered our headers were being slightly altered.

The “bug” word escalated things to almighty Google (who had recently acquired Postini). It wasn’t a “spammer or not” issue anymore. Bugs are technical things that can be solved. Not ethical things that are hard to define.

The outcome was that Postini discovered they were slightly altering email headers in a way that was causing false spam filtering by another system (Trend Micro) at the client:

“After investigation from our Engineering group, we have in fact determined this to be a bug with the way our system handles the escaping of characters in e-mail addresses. It seems that we are improperly escaping both the ‘=’ and ‘~’ symbols with a backslash, which your Trend Micro system is rejecting. This has been documented as a bug and out Engineering group is actively working to resolve the problem. Unfortunately I do not have a specific ETA as to when this will be resolved in production.”

The Trend Micro connection explains why some of our clients get past Postini just fine, but others don’t.

If other ESPs are having problems with Postini, we hope this helps in some way. Mysterious Postini issues can make you pull your hair out of your head and kick your dog out of anger. Knowing it’s a bug might make you feel better. You might also investigate whether or not your client is using Trend Micro, or if the bug is causing problems with other systems too. Ahem, feel free to post updates here for the greater good (of balding ESP engineers).