Persado founder and CEO Alex Vratskides loves it when his sales team arranges trials of his company’s email marketing software.

The challenge to potential customers: Use the product, called Persado Go, to suggest subject lines for several existing campaigns—and then test the results head-to-head against messages sent using ideas from the human creative team. “We almost always outperform their best efforts,” Vratskides tells Fortune. “After we provide that proof in two or three campaigns, it’s a pretty straightforward sales proposition.”

Persado’s software uses machine learning—and a huge database of campaign performance information about millions of messages—to identify language that is most likely to strike a chord with specific demographic groups. It can suggest words most likely to elicit an emotional response to a message, and warn marketers if certain words are culturally sensitive or whether a local holiday might interfere with acceptance rates.

Given all the hoopla about mobile apps and messaging, you’d be forgiven for overlooking email’s ongoing influence as a way to reach potential and existing customers. But a majority of big brands (60%) are increasing their email marketing budgets this year. Almost three-quarters of marketing teams, however, consider email as one of the most consistently effective ways to reach their target audience. Approximately the same number are looking for ways to make their messages more personal.

“Choice of words really matters,” says Vratskides. “Imagine if systematically you could get to the words that resonate most with the audience.”

Persado, which received $15 million in backing from Bain Capital in 2013, was spun out of mobile marketing agency Upstream. Its technology is used by approximately 70 large companies, including financial services firms American Express and Citi c , e-commerce company eBay ebay , software giant Microsoft msft , and retailers Neiman Marcus and Sears shld .

While the company supports more than 20 languages, the large majority of the campaigns shaped by Persado’s software are focused on U.S. consumers, Vratskides says.

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Hospitality company Caesars Entertainment czr found that test emails sent with subject lines generated by Persado’s software were far more likely to be opened than those crafted by its marketing team—in some cases, the open rate increased by 24%, according to Chris Jenner, vice president of email marketing for Caesars.

“I thought the proof of concept was a fluke, but I was blown away by the results,” Jenner admits to Fortune.

Caesars has used Persado’s software since early 2015, for close to 50 campaign tests.

“There is a cost to leverage the tool, but it is extremely reasonable,” he says.