"Email is still very much king," said Henri Makembe, a partner at Beekeeper Group, a digital public relations company in Washington. "When I have your email, I can get your attention."
Despite all the new technology that has emerged since the last presidential election, email is still seen as one of the most effective fund-raising tools a campaign can employ.
Martha Patzer, former deputy email director of Obama for America, said the average donor for the Obama campaign gave money three times. Part of the reason, Patzer said, was the tone of the emails that were sent out.
An email with the subject line "Hey," which mirrored email correspondence that people might get from a friend, was one of the most successful. In one version of the email, signed "Barack," the President asks the recipient, who is addressed by his or her first name, to "chip in $5 or more, and let's go win."
And win, they did.
Digital donations, through email and other outreach methods, accounted for more than $500 million in campaign contributions -- and two-thirds of that was via email.
"As people receive more and more media and messages, we're trying to create more personalized experiences for ourselves," said Patzer, who is now a vice president at 270 Strategies, a digital strategy group that works with political campaigns including the Ready for Hillary super PAC. "How many times a day do you use the subject line 'Hey' or 'Quick Question'? We wanted it to feel approachable, casual, inclusive, really authentic."
A big part of the Obama campaign's advantage in 2012 was that it built its list of email addresses over the course of years -- unlike today's presidential candidates, who will be scrambling to build their lists. Democratic front-runner Hillary Clinton recently gained access to about 4 million email addresses culled by the Ready for Hillary super PAC. But that list still pales in comparison to the approximately 20 million email addresses the Obama campaign is said to have amassed.
For their part, Republicans, who have been viewed as less digitally savvy than Democrats in their political strategies, say they see the value in email as well. In 2012, Mitt Romney's campaign, in conjunction with the fund-raising arm of the Republican National Committee, raised about $130 million using an email list of about 6 million supporters.
Tim Cameron, the chief digital strategist at the National Republican Senatorial Committee, said he expected the use of email by Republican candidates to increase in 2016.
"The most valuable asset that a campaign has is a great email list," Cameron said.
Candidates can increase the size of their email lists by gathering addresses from supporters at rallies, through online merchandising or ticket sales, or using social networks like Twitter, Cameron said.
The trick for many candidates will be to maintain the sanctity of their lists. Former Republican presidential candidates have been criticized for selling their lists to online marketers hawking everything from erectile dysfunction products to cancer cures. In May, Mike Huckabee defended the practice, saying it was no different than CNN airing ads alongside programming.
Another reason campaigns love email is that it remains one of the most personal forms of communication a candidate can have with a supporter.
"When you have somebody's email address, you know you can get in touch with them," said Joe Rospars, the founder and chief executive officer of Blue State Digital, the digital strategy team that worked with the Obama campaign in 2008 and 2012. While posting content on social media like Twitter and Facebook is important, "not all of those people are going to see everything that you post."
Email is also consistent, said the NRSC's Cameron.
"Over 90% of people check their email every day," he said.
In contrast, social media companies like Facebook often tweak what type of content users can see.
"With email I don't have to worry about a CEO of a company changing an algorithm."
There is also no guarantee that someone who uses a social media platform today will still use that platform in a few years, Cameron said.
Jeff Bechdel, a spokesman for the conservative America Rising PAC, said contacting people via email today means being able to access them whether they are on their phones, tablets or other computers. And since most supporters who get emails have explicitly allowed the campaign to contact them that way, "those are the people who are going to donate, those are the people who will turn out to volunteer," Bechdel said.
Candidates, however, will have to measure their use of email in order to ensure that type of success, said Dan McSwain, who worked on the Obama campaign in 2008.
"Email is sort of treated like a cash register," McSwain said. "Where email is seen as the quick trip to the bank, the risk is that overly relying on that kind of transactional relationship weakens that sort of opportunity."
And the last thing a candidate wants is to be ignored, especially on email.