Postini as an uncommunicative bouncer

One of the spam filters in our design and spam testing tool is Postini. This filter is now owned by Google, and is notoriously difficult to get past.

Postini is like a bouncer at a nightclub who not only won’t let you in, but won’t even tell you if it is because of your shoes, or because you don’t have enough women in your group (not that it ever happened to us of course).

Recently a Campaign Monitor customer, Dave Green, ran into this problem with his campaign, and was able to do some testing to find out why Postini was blocking his email. His results in the end were useful, but surprising:

What I have found (8 tests later) is that I had to chop up the larger images into a much smaller sizes both in dimensions and also in byte size . The largest kb is 14.3 and found I had to chop files into physically smaller dimensions for Postini to pass it.

Basically it boiled down to trial and error, chopping images up trying to optimise them without causing major loss of image quality and re-testing.

Spam filters (understandably) don’t reveal exactly how they work, but it seems clear that Postini is placing a lot of importance on the balance between images and text (or HTML) in an email. Dave also pointed out that our simian friends at MailChimp have made the same finding.

Given how prevalent image blocking is in email clients, it makes sense not to rely on big images in your emails, but this is one more reason to be cautious.

We can add image dimensions and image file size to our list of factors impacting spam filtering. Here’s a reminder of some other things to watch out for:

  1. Avoid repeatedly sending messages to full or invalid mailboxes. You can do this by tweaking your bounce handling settings for each subscriber list.
  2. Minimize the use of these words and phrases in the subject line, message body, sender address, and reply-to address:
    • Use of the word Free (although “free” tends to have more leeway than most other trigger words), $$, XXX, sex or !!! (any excessive punctuation)
    • Subject contains “Double Your”, “?”, “For Only” or “Free Instant”.
    • Email contains at least 70 percent blank lines
    • The from field appears to not contain a real name, ends in numbers or contains the word friend.
    • The email claims not to be spam
  3. Monitor new subscribers in your lists. Set suspicious “spamflag” addresses such as “abuse@” or “spam@” as Inactive subscribers unless you know the subscriber is legitimate.

There are no shortcuts or certain ways to avoid spam filtering, but these guidelines can help reduce the risk factors. Filter providers are less and less likely to provide helpful information about their products, so we will be relying on trial and error even more. Thanks to Dave for his great work in this case!

More important than any of these tip, tricks and tests is understanding that spam filters are not the biggest issue. The key to modern email marketing is understanding that relevance beats permission. Even if your email is being delivered into the inbox, you can still get spam complaints if you are not ensuring relevance.

  • Cameron Compton

    Just to clarify, when you say “Email contains at least 70 percent blank lines” are you referring to empty lines in the html, like a designer would use for code formatting?

Want to improve your email marketing? Subscribe to get tips on improving your email marketing delivered to your inbox.

Join 200,000 companies around the world that use Campaign Monitor to run email marketing campaigns that deliver results for their business.

Get started for free