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DKIM: How one email deliverability tweak improved Gmail open rates by 400%.

Spammers try to trick internet users by placing a domain in the “from” field that they aren’t associated with (like a scam artist pretending to send from “”). To combat this, DomainKeys Identified Mail (DKIM) allow marketers who send campaigns from “” to actually prove that the email came from “”. Talk about deliverability with an email guy/gal and this topic is bound to come up. But does it really work?

The Goods
To find our answer, we compared fall and summer email open rates for two groups. The first group setup DKIM at the end of summer 2013. The other group did not.

We chose to compare the difference between fall and summer open rates because seasonality has a huge impact on email performance. Email performance also varies between resorts. By comparing just the improvement in email opens from summer to fall we were able to look at just the effect DKIM had (or didn’t have) on resort email performance. Here’s how it looked for the big-four providers (Yahoo!, Gmail, Hotmail, AOL) that account for so many of the email addresses in resort databases:

We’ll ignore the obvious result for a second and focus on the other three. First, Yahoo! and Hotmail both showed better open rate improvement with DKIM than without. Our sample was relatively small so it’s not conclusive, but the results are encouraging. With AOL, there wasn’t a significant change with DKIM versus without.

The clear winner, however, was Gmail. From summer to fall, resorts without DKIM saw a 29% jump in opens. Resorts that did setup DKIM saw a 492% jump in opens. Rather than jumping ahead of the competition in terms of overall performance, DKIM actually helps marketers get Gmail open rates on par with other providers. Without DKIM, open rates are incredibly low for recipients with Gmail addresses.

What This Means
When we first pulled these numbers, we expected DKIM to make a difference, but after seeing a 400%+ jump in open rates with Gmail, we almost couldn’t believe our eyes. As we analyzed more and more resorts, however, the trend held. In fact, we started using Gmail open rates to verify that DKIM was setup and working properly.

The bottom line is this: without DKIM, you are missing many, many opens from Gmail users. Gmail seems to be much more strict with their spam filters and, unless you prove that your campaigns came from the same domain you’re claiming in the “from” field, they simply aren’t going to get through.

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