With the simple tap of the “send” button one day last week, Donald Trump collected $3 million in campaign contributions—as much as he did in the entire month of May. He had asked for donations of $10 or more, with the promise of chipping in $2 million of his own money to match those that arrived.

That one-day haul from Trump’s first fundraising appeal is early evidence of the digital magic it takes to fill campaign coffers Bernie Sanders-style—millions of people, each giving a few bucks.

Yet that was just one email. Success demands repetition.

The presumptive Republican nominee must now make the case that he needs money, after months of boasting that he can pay his own way. And his campaign also is failing in what could be called “the art of the email.” One analysis found that 74 percent of his first fundraising requests landed in spam folders.

Still, if Trump can reap millions of dollars from each pitch, that could help him solve an urgent problem: He’s being crushed by Democratic rival Hillary Clinton’s well-honed finance machine, which pulled in 10 times as much as he did last month. Campaign money pays for the advertising and employees needed to find, persuade and turn out voters on Election Day.

Trump’s national finance chairman Steven Mnuchin said the campaign was “overwhelmed” by reaction to the first online fundraising appeal. “This is now going to become a daily effort,” Mnuchin said.

Since that initial email, the Trump campaign has sent at least four more solicitations, including one Sunday from chief strategist Paul Manafort touting the fundraising success of the week and urging supporters to keep up the momentum.

Trump’s partnership with the Republican National Committee also pays special attention to the small donors who typically give online. They have a joint account called the Trump Make America Great Again Committee that has sent two dozen emails in the past month. “Contribute $100, $50, or even just $25 to show you’re ready to keep winning!” one missive asks. Each donation is divided, with 80 percent going to the Trump campaign and 20 percent to the RNC.

As successful as Trump’s first fundraising email seems to have been, Tom Sather, senior director of research at the email data solutions firm Return Path, said the candidate could have done better. The firm measures emails much the way Nielsen measures television viewership, by extrapolating from a large panel of study participants.

Just 8 percent of the email recipients opened them up, according to Return Path’s analysis. The campaign’s stunningly high spam rate of 74 percent reflects a lack of email marketing sophistication, Sather said. For example, the campaign switched domain names recently, tripping up spam filters, and Trump may be buying email lists of people who don’t want to hear from him.

By contrast, Clinton’s spam rate on fundraising emails is typically about 5.7 percent, and her rate at which people open the emails holds steady at about 14 percent, Sather said.

“It will be interesting to see how he gets better at this, or if he continues to flounder,” Sather said. “There is an art and a science involved.”

Trump has begun leveraging his social media fan base for cash. In a sponsored Facebook post on Tuesday, Trump asked for donations after a reminder that he is new to fundraising. “I did a good thing during the Republican primary. I didn’t ask my supporters for a single dime. Not one.”

Trump’s early pitches seem designed to tackle the problem of how it looks when a rich guy starts asking for money, Republican strategists said.

“He has built rapport with voters. So if he says he now needs their money, they’re more likely to trust that he does,” said John Thompson, the digital director for Ted Cruz’s Republican presidential campaign.

Dale “Boomer” Ranney, a South Carolina-based volunteer for Trump, recently gave $125 to the campaign. She pointed out that if all of Trump’s 9 million Twitter followers did so, he would blow past the $1 billion that Clinton and her allies are expecting to amass.

Trump’s grassroots supporters will “give what they can because they believe so much in him,” she said.

As Sanders proved, online fundraising can chip away at an opponent’s financial advantage. Sanders raised $6 million in 24 hours after winning the New Hampshire primary simply by declaring in a televised victory speech that he was “going to hold a fundraiser right here, right now, across America.”

“If they really focus on it, they could raise $300 (million) or $400 million online,” said Barry Bennett, a former Trump adviser who helped Ben Carson raise tens of millions of dollars online for his presidential bid.

Bennett and Thompson said they could imagine ways for Trump to raise big online.

For example, the campaign could design fundraising raffles with the prize of meeting Trump or touring his airplane or his glitzy properties, Thompson said.

“He has a luxurious lifestyle,” he said. “If I was in their shoes, and he agrees to it, I’d leverage that.”