I’ve mentioned here before that I can usually tell when the big ISPs are making changes to their spam filtering as that ISP dominates my discussions with current and potential clients and many discussions on delivery mailing lists.

The last two weeks the culprit has been Yahoo. They seem to be making a lot of changes to their filtering schemes right at the busiest email marketing time of the year. Senders are increasing their volume trying to extract that last little bit of cash out of holiday shoppers, but they’re seeing unpredictable delivery results. What worked to get mail into the inbox a month ago isn’t working, or isn’t working as well, now.

Some of this could be holiday volume related. Many marketers have drastically increased their mail volume over the last few weeks. But I don’t think the whole issue is simply that there is more email marketing flowing into our mailboxes.

As I’ve been talking with folks, I have started to see a pattern and have some ideas of what may be happening. It seems a lot of the issue revolves around bulk foldering. Getting mail accepted by the MXs seems to be no different than it has been. The change seems to be based on the reputation of the URLs and domains in the email.

Have a domain with a poor reputation? Bulk. Have a URL seen in mail people aren’t interested in? Bulk. Have a URL pointing to a website with problematic content? Bulk.

In the past IPs that were whitelisted or had very good reputations could improve delivery of email with neutral or even borderline poor reputations. It seems that is no longer an effect senders can rely on. It may even be that Yahoo, and other ISPs, are going to start splitting IP reputation from content reputation. IP reputation is critical for getting mail in the door, and without a good IP reputation you’ll see slow delivery. But once the mail has been accepted, there’s a whole other level of filtering, most of it on the content and generally unaffected by the IP reputation.

I don’t think the changes are going to go away any time soon. I think they may be refined, but I do think that reputation on email content (particularly domains and URLs and target IP addresses) is going to play a bigger and bigger role in email delivery.

What, specifically, is going to happen at Yahoo? Only they can tell you and I’m not sure I have enough of a feel for the pattern to speculate about the future. I do think that it’s going to take a few weeks for things to settle down and be consistent enough that we can start to poke the black box and map how it works.