A wave of new startups is taking aim at corporate email, promising to disrupt the mode of communication that office dwellers love to hate. But the old inbox is putting up a fight.

Slack, which lets employees keep track of projects or topics via open, searchable feeds, was released to the public in February and recently raised $42.75 million in fresh funding. It counts more than 96,000 active daily users, including some at companies like Airbnb Inc.

Asana, led by former engineers at Facebook Inc. and launched in 2011, promises "teamwork without email" by allowing users to create, assign and comment on specific tasks. It has won devotees at tech players like Uber Inc. and Foursquare. The company said it has thousands of paying customers but didn't disclose the exact number of users.

Despite the praise of these early adopters, managers and digital experts say technological and social hurdles remain. For now, they say, the promise of a world without email is a fantasy for the majority of corporate workers.

Michael Morgenstern

Email usage is "exploding," said Gloria Mark, a professor at the University of California, Irvine, who has studied how workers interact with the technology. Not only are workers wading through ever more clogged inboxes, they're also checking frequently, an average of 74 times a day, according to coming research from Prof. Mark.

The result: distracted workers. A 2012 study from Prof. Mark and several colleagues found that workers who were cut off from their email focused for longer periods of time, switching screens less frequently, and were less stressed, as measured by heart-rate monitors.

"It's really out of control," she said of email usage.

Some small companies say they've been able to kick the habit. Treehouse Island Inc., an online coding school based in Portland, Ore., said most of its 72 employees receive two to three emails a day. Last year, the company shifted internal communication to three different programs, including the instant messaging service HipChat. Workers are happier and getting more done, said Chief Executive Ryan Carson.

But other companies' experiments haven't fared as well, either because of ingrained habits or because the new systems became as overwhelming as their predecessor.

Myplanet Internet Solutions Ltd., which builds applications and e-commerce platforms, started using Slack last August. Slack's feeds, called "channels," feature messages, files and accompanying comments, images and video. Users can also pull in outside streams of information—like Twitter feeds—tag colleagues to alert them to a post and send private messages.

Previously, Myplanet's 83 employees were toggling between email, video-chat program Skype, and social network Yammer. After Slack was introduced, only about half the staff ended up using it. Many clung to Skype, and email usage didn't decrease, according to Yashar Rassoulli, the company's technology director.

A Myplanet survey revealed that employees felt better informed about what other people in the company were doing after using Slack but also felt more distracted by all the conversation.

Technological quirks also bothered, and in some cases alienated, Myplanet employees. Workers would lose track of conversations; while a fresh email always lands at the top of someone's inbox, a new message on an old Slack conversation wasn't automatically placed at the top of the feed, said Mr. Rassoulli. And visually impaired employees at the company found that Slack didn't sync with the programs they used to work via computer.

The company stopped using Slack in April.

A spokesman for San Francisco-based Slack said it has made enhancements to the program since Myplanet stopped using it, and that new messages always go to the top of the feed. It also plans to eventually explore making the program compatible with screen readers for the visually impaired, the spokesman said.

Stewart Butterfield, Slack's CEO, acknowledged the product's limitations, especially when it comes to bigger companies, where there is an incredible amount of information to be wrangled. He said the product works best with boundaries, like a smaller team—50 to 200 people.

"If you try to bring in all the information that's relevant to 10,000 people, there's no good way to manage it," he said.

Some big businesses say they are hesitant to embark on companywide anti-email campaigns. Neither Slack nor Asana have converted entire Fortune 500 companies into clients. Instead, small teams at large companies like Adobe Systems Inc. use Slack, according to Mr. Butterfield. He said teams often start using the product without gaining formal corporate approval. Adobe declined to comment.

Both Slack and Asana are rolling out functions intended to ease their integration at bigger companies. Mr. Butterfield said it will be years before an entire Fortune 500 company could use Slack on an organizational level.

Whirlpool Corp. is transitioning to the Google suite of products, including Google chat and Google Drive, in the hopes of showing its approximately 70,000 employees that email isn't the only form of communication, said Lynanne Kunkel, the company's vice president of global talent development. Still, Whirlpool doesn't have a commitment to eliminating the form of communication, Ms. Kunkel said.

"Email is an important part of my portfolio," she said.

Workers who spend a lot of time communicating with the outside world might find it difficult to shift away from email in the near future. Clients might not have a desire to adopt new technology, and even if they did, technological and organizational barriers would probably prevent them from gaining access, said Ted Schadler, a technology analyst with Forrester Research Inc.

Slack said it's working on restricted accounts, which would let clients or contractors use certain services but keep them out of other areas of the program.

Mr. Schadler said corporate workers shouldn't expect email's demise any time soon.

"People use email because it's the best, most reliable way to get to anybody on the planet and none of these other tools let you do that; none of them," he said.

Write to Rachel Feintzeig at rachel.feintzeig@wsj.com