When Google introduced a new filtering system for its email service Gmail last month, I suggested that it might cut people’s exposure to promotional emails (see “Marketers Must Hate Google’s New People-Focused Inbox”). Figures released today by email marketing company MailChimp today show that is already happening, depsite the feature likely being used by only a fraction of Gmail users (here’s how to activate it).

Google’s new system automatically filters incoming emails into several different “tabs” of a person’s inbox. Anything that looks like a marketing email to Google’s algorithms ends up in the “Promotions” tab (where Google has also started to display a new form of ad).

Matthew Grove of MailChimp writes in his post that the rate at which Gmail users opened marketing emails sent via his company’s service was 13 percent or higher for a typical weekday before the new feature was introduced. A week after the feature debuted, open rates were under 12.5 percent.

Grove, who is writing for an audience of email marketers, concludes that Gmail’s new design is making marketing emails less effective but that the shift isn’t yet “dramatic.”

I’d add to that that there’s a good chance that shift will become more pronounced, since it is unlikely that every Gmail user has switched on the new filtering feature.

Emails filtered by the promotions tab targets are legitimate. People have done something to start receiving them, and are provided with ways to opt out. However, the challenges of dealing with the flood of such messages will likely drive Google’s new solution to be increasingly popular.

Meanwhile, Grove’s post signals that the marketing community is set to enter something of an arms race with Google, similar to that between the creators of spam email and the designers of spam filters:

“I’ve heard a lot of people asking how they can get out of the Promotions tab and into the Primary tab. There aren’t any good answers here, because Gmail is really good at what they do… we’re definitely testing the new inbox and trying to figure out how it works.”