The billionaire running for president now seeks to convince millions of Americans to give him money.

Donald Trump speaks at a rally in San Diego on May 27, 2016 in San Diego, California. (Photo by Spencer Platt/Getty Images)

Donald Trump raised $3 million online after sending one email last week asking for campaign contributions and promising to chip in his own money. That's as much as he raised in the entire month of May.

That one-day haul shows he might have the digital magic it takes to fill his coffers Bernie Sanders-style, from millions of people each giving a few bucks.

But Trump's campaign has struggled to master what could be called "the art of the email." A study by email data solutions company Return Path finds that about 74 percent of Trump's first email solicitations ended up in people's spam folders.

Trump is being drastically out-raised by Democrat Hillary Clinton.

(Copyright 2016 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.)

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