The billionaire running for president now seeks to convince millions of Americans to give him money.
With the simple tap of the "send" button one day last week, Donald Trump collected 3 million US dollars (£2.2 million) in campaign contributions - as much as he did in the entire month of May.
He had asked for donations of 10 US dollars (£7.55) or more, with the promise of chipping in 2 million US dollars (£1.5 million) of his own money to match those that arrived.
That one-day haul from Mr 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 dollars.
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 is also failing in what could be called "the art of the email". One analysis found 74% of his first fundraising requests landed in spam folders.
Still, if Mr Trump can reap millions of dollars from each pitch, that could help him solve an urgent problem: He is 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.
Mr 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," he added.
Since that initial email, the Trump campaign has sent at least four more solicitations, including one on Sunday from chief strategist Paul Manafort touting the fundraising success of the week and urging supporters to keep up the momentum.
Mr 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% going to the Trump campaign and 20% to the RNC.
As successful as Mr 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 by extrapolating from a large panel of study participants.
Just 8% of the email recipients opened them up, according to Return Path's analysis. The campaign's stunningly high spam rate of 74% reflects a lack of email marketing sophistication, Mr Sather said.
For example, the campaign switched domain names recently, tripping up spam filters, and Mr Trump may be buying email lists of people who do not want to hear from him.
By contrast, Mrs Clinton's spam rate on fundraising emails is typically about 5.7%, and her rate at which people open the emails holds steady at about 14%.
"It will be interesting to see how he gets better at this, or if he continues to flounder," Mr Sather said. "There is an art and a science involved."