The podcast Serial, a true crime story told in tantalizing weekly installments by the producers of This American Life, has captivated listeners on the internet. More surprisingly: so has its advertising.

This 20-second ad for the email service MailChimp, the sole sponsor of Serial, runs at the beginning of each episode:

(Molly Fitzpatrick of Vocativ cut the audio and put it on SoundCloud.)

Fans have embraced the ad almost like it’s one of the podcast’s many compelling characters. Jokes about the pronunciation of Mail… Keemp? have developed into their own meme. (For the record, MailChimp says there is no official spelling of the now-famous mispronunciation, but the company did register MailKimp.com.)

The ad was written by MailChimp and produced by Serial‘s Dana Chivvis and Julie Snyder. It’s a sly takeoff on the aural aesthetic of public radio reportage mastered by shows like This American Life: Hearing—but not seeing—each person forces you to picture the scene. Subtle touches, like the woman at the end who says, “I use MailChimp,” at a distance from the microphone, adds a richness that’s only possible on radio.

But maybe that’s overanalyzing things. The ad has also succeeded merely by association with a beloved program, currently the most popular podcast on Apple’s iTunes amid a broader renaissance of podcasting in the United States. Listeners binging on several episodes at a time become familiar with the ad by repetition.

Regardless of the reasons for its success, the MailChimp ad is clearly one of the biggest marketing coups of the year, with a cultural resonance eclipsing many Super Bowl ads that cost millions of dollars to air on TV. To get the story behind the ad, we spoke with MailChimp’s marketing director, Mark DiCristina, about the concept and what might be next for MailChimp and Serial. (This interview has been condensed and rearranged for clarity.)

Mark DiCristina

So, are you listening to Serial?

Oh yeah. I love Serial. I’m super excited about it. That’s one of the things that’s important to us. We don’t advertise on too many podcasts, but the ones that we advertise on, they’re podcasts that we like ourselves, that we’re excited about listening to. When they approached us about sponsorship—we had been sponsors for This American Life for a while—as soon as they told us what the concept was, I couldn’t wait for it to start. I’m just as obsessed with it as everyone else.

How collaborative was the process to create the ad between you and the Serial producers?

Pretty collaborative, actually. We gave them the copy for the ad, and then they made the ad itself. Dana Chivvis took the copy and went out in the streets of New York and recorded a bunch of different people reading the ad and then cut it together the way that she did.

That last part of the ad with the woman saying she actually uses MailChimp was unscripted, right?

As far as I know! [laughs] I mean, that was not in the copy. I was surprised as much as anyone else. It worked out really nicely, but we didn’t plan it that way.

The fact that you can only hear the audio and not see the people—it adds a level of intrigue.

For sure. I think the variety of voices and accents makes it interesting. I think the fact people hear it over and over before each episode probably contributes to it.

And, quite frankly, I just think the fact that the show is so good, a lot of that rubs off on the ad itself. Part of what people are responding to when they’re responding to the ad is really their affection for the show itself. We’re beneficiaries of that excitement, but I don’t think that it’s necessarily a result of the ad itself.

Can you measure the success the ad has had, in terms of web traffic or Twitter activity?

There’s definitely more people talking about it on Twitter. Hundreds and hundreds of people everyday are using the #MailKimp hashtag, which is pretty funny. This time of year for us, there’s generally a surge in the amount of email people send, the amount of people signing up for MailChimp. That’s kind of an organic increase for us. We didn’t expect the ad itself to translate into people signing up for sending with MailChimp. It was more for brand awareness.

Is the only way to measure ad impressions through iTunes downloads?

Yeah. Podcast ad buys are typically like any advertising on the internet. They’re purchased on a CPM [cost per thousand] basis, and the CPMs in this case are based on downloads.

With the success of this ad, will you start doing more advertising on other serialized shows and podcasts?

Possibly. We’re interested in supporting content and content creators who are doing really interesting stuff. We had been sponsors for This American Life for a while; that certainly fits the bill. Serial fits the bill, and we’ll definitely be back with Serial. Although I don’t know if we want to try to recreate the magic that happened with the ad. We’ll probably just want to do something completely different.

So if there’s a second season of Serial, there will be a second “season” of the MailChimp ad?

Definitely.