When Sparrow agreed to be acquired by Google in mid-July, users cried out in anguish. They were losing their favorite mail app for Mac — perhaps the first app to truly prove there's a market for an alternative to Gmail on the web and Apple's Mail app on desktop. As Sparrow drifts slowly towards the abyss (though its makers promise critical bug fixes), a few competitors like Postbox, Mailplane, and .Mail have entered the limelight. Considering how much time people spend sending emails every day, why are there so few mail apps for Mac, and who's left to bid on Sparrow's abandoned users?

Postbox, perhaps the most technically capable third party email client, saw an uptick in downloads as soon as Sparrow and Mozilla's Thunderbird mail client announced plans to cease development. Postbox was Sparrow's chief premium competition, fitting in a nice niche between Apple's drab Mail app and Sparrow's colorful and modern app. But the climb to the top is long and hard. "Microsoft and Apple also offer free email clients and have powerful distribution and OS integration advantages," founder Sherman Dickman says. "It's difficult to compete with free... One must provide a set of unique features that are flawlessly executed."

"One must provide a set of unique features that are flawlessly executed."

Postbox includes a few key features its competitors lack, like Google Calendar and Evernote integration, template responses, and the ability to update your status on social networks. "Email lacks the virality and network effects of social offerings," Sherman says. "It's simply a different interaction model, and thus, a slower growth business. Perhaps this is another reason why we don't see more startups in the space. There are a lot of moving parts, and thus, it's much more difficult to engineer an email client than many other apps," he says. "It's a very tough market.... You see mostly modest increases and decreases, but not a lot of hockey stick growth for startups in this space."

Part of the challenge is that building a mail client is very hard. Sparrow architect Dinh Viet Hoa spent close to ten years of his free time on the mail engine that powered Sparrow. "You have to put in a huge amount of work," Dinh says. He had to learn a panoply of IMAP protocols and specifications, build his own IMAP parser to make requests from mail servers, a MIME parser (in case the server doesn't abide by IMAP specs), and the tools to make connections to IMAP servers asynchronous. "When you have these basic things down then you can start building a client," he says. After that comes offline support, POP3, attachment-handling, and cache optimization. "At this point, you don't even have support for filters and spam," Dinh says. Some suggest that even after all that hard work, Sparrow wasn't making enough money to sustain even a handful of full-time employees.

"The hardest part about building Mailplane is getting along with Google and Apple at the same time."

Mailplane, a hybrid desktop and web app, didn't see a difference in sales once Sparrow made its announcement. "We haven't seen a huge uptick yet, but we've got positive feedback and hear from users who are looking for an alternative," developer Lars Steiger says. Mailplane relies on Gmail but incorporates many features you'd find in Sparrow or Apple's own Mail app."The hardest part about building Mailplane is getting along with Google and Apple at the same time," Steiger says.

While Mailplane needs both Apple and Google to survive, it's also up against them, as well as other webmail providers like Microsoft. Many Sparrow users have fled to web clients like Microsoft's brand new Outlook, which offers both a simple and beautiful user interface. Outlook lead Brian Hall says, "increasingly the line between what's an app and a service continues to blur." As internet access becomes omnipresent, the benefits of a local mail app start to run thin. Many of Windows 8's home screen apps only work if you have a persistent internet connection.

Outlook, Mailplane, and Postbox have been around for a while, but new entry .Mail also recently got off the ground — the first Mac mail app we've heard of since Sparrow. .Mail is inspired by Sparrow visually, but began as a concept demo with no mail engine to back it up. "I think the toughest parts will be definitely the scaling and performance," designer Tobias Van Schneider says, who just recently found some developers to code his app. Van Schneider and company have a long way to go to catch up to the competition, but the ravenous response to his concept design prove there's a market for a design-centric mail client.

While mail app developers are indeed battling each other for Sparrow's almost-vacant throne, their main adversaries seem to be Google and Apple themselves, which have enormous amounts of resources and the ability to give away their products for free. Mailplane's Steiger says, "Google is constantly changing their apps and Apple is getting stricter all the time... It's always hard to battle the mothership."