Filter complexity


URLBlockingForBlogDuring the Q&A last week, I mentioned an example of a type of filter trying to demonstrate how complex the filters are. There was some confusion about what I was saying, so I thought I’d write a blog post explaining this.


This story came from another deliverability person, let’s call her ESPer. One of their customers (Customers) is using a 3rd party service that provides tracking links (Tracker). Tracker sent email to their customers saying that mails with more than 3 links were getting blocked.

It has come to our attention that Google has recently started flagging emails with multiple tracked links as suspicious or malicious. For example, if you have an email with more than 3 links (including any in your signature) and have Tracker link tracking turned on, recipients who use Gmail may see your message flagged with a warning. If your email contains 3 or fewer tracked links then you will be unaffected by this issue.

This triggered some Customers to call the ESP and start asking if Google was blocking mail with 3 or more links.

The Investigation

Multiple ESP folks checked their systems and found no correlation between multiple links in an email and bulk foldering at Gmail. I checked my Gmail account and a number of emails in my inbox have 4 or 5 or 6 links in them. None with the Tracker tracking cookie, though.
In an effort to test this a little more, I tried to sign up for a free account with the Tracker to do a little more checking. Tracker is used through an add on for use in Firefox, but it’s unsigned so I decided not to install it. It’s probably not malware, but if they can’t be bothered to sign their Add-on, I’m not going to risk installing it on my machine, even for my readers.

What we know

  1. Gmail is blocking mail with 3 or more links with one that is a Tracker link.
  2. Remove the Tracker link then mail goes to the inbox.
  3. Send with less than 3 links and a Tracker link then mail goes to the inbox.

What we speculate

One of the customer of Tracker is sending spam with 3 or more links plus the tracking links. Google has identified this mail as a problem and is blocking mail that has the same characteristics.
Removing the Tracker link should get the mail into the inbox.
Removing links so there are less than 3 links should get the mail to the inbox.

What this tells us

Filtering is complex. Like Really Really Complex. It’s not the presence of the tracking URL, it’s the presence of the tracking URL and 3 other URLs. Generally when we here at Word to the Wise try and test “what’s wrong” we’ll start removing URLs to see if one particular URL is causing a problem. In this case, that testing would have led us to an erroneous conclusion. We might find one URL “responsible” but only because we’d lowered the total number of URLs under 3.
I’ve been telling people and clients that filters are complex. More than 3 URLs + a specific URL is something that people wouldn’t normally identify as a filter criteria. But the neural net / machine learning / AI filters in use at Gmail noticed that mail with a particular number of links plus the Tracker link aren’t wanted by the recipients. The filters then started blocking mail selectively based on those criteria.
Filters aren’t magic, but sometimes the complexity makes them seem like it.

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By laura

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