In many ways, Superhuman is your archetypal Silicon Valley darling. It was founded by a brilliant Cambridge graduate who sold his first company to LinkedIn in 2012 then quit to start another one. It has attracted a cult following among early users, whose lives it promises to revolutionise (and who often insist it really has done so). And now it has scored a $33m investment from Andreessen Horowitz, the influential venture capital outfit that was an early backer of Twitter, Instagram, Facebook and almost every other major startup going since it was formed in 2009.

But, in other ways, Superhuman is nothing like its peers. For one thing, it charges for what it makes: an eye-watering $30 a month. For another, it’s taking the opposite route to the growth-at-all-costs mindset of so many startups: the Guardian first tried to sign up in 2016, and sat on the waiting list for almost three years before giving up and pulling strings to get VIP access. Oh, and it isn’t building an AI blockchain for using augmented reality to run your biotech, or anything trendy like that. No, it makes an email client.

“From a customer’s perspective the pitch is really simple,” says Rahul Vohra, Superhuman’s founder and chief executive. “We’ve built the fastest email experience in the world. Our customers get through their inbox twice as fast as before, get to the emails that matter faster, and many of them see inbox zero for the first time in years – which, as you can imagine, is pretty life changing.”

That speed comes from two places, Vohra says. Well, actually it comes from “many hundreds of product decisions that have come together to make people go faster”, but they can be lumped into two categories: firstly, there’s the technology.

“Everything, even search, happens in 100ms or less,” Vohra says. Superhuman uses canny tricks on Chrome to make the most of all the processing power a modern computer has, he explains, and to ensure that every email is available, and searchable, offline.

And then there’s the design. The whole service is built from the ground up for people who live their life in the inbox, and it shows. “It’s aggressively keyboard-shortcut focused,” Vohra says, as he talks through the client. “You do everything from the keyboard.”

Keyboard shortcuts are nothing new, but Superhuman practically shouts at you if you touch your mouse. Want to compose an email? Hit C. Need to CC someone in? Cmd-Shift-C will do that. Need to jump to a labelled email? Cmd-K will bring up a prompt, then just type the label name and hit enter. If you ever think you need to move your mouse to do something, a tooltip will pop up at the bottom explaining how to do it quicker from the keyboard.

And then there’s a third reason: that $30 price-tag, and the focus it brings. It’s certainly an eye-catching promise. “Email goes 24-Karat”, the New York Times recently proclaimed.

“In the software industry, we have let ourselves fall into the trap of bad business models: of everything being ad supported,” Vohra says, when I ask him whether a $360-a-year service can really “democratise productivity,” as he argues. “I believe in the most honest and successful business model in the world, and that is: if you value something, you should pay for it. And for our users, it’s incredibly good value. You spend $30 for hours back a week.”

Focusing only on those who care about email enough to spend hundreds of pounds improving it helps. It lets the company require a moderately new laptop, for instance, to free it from the need to support slower machines. It also enables one of its more eye-catching requirements: a one-on-one introductory session with all its users, who must demonstrate their workflow to an expert at Superhuman over a video call, before receiving personalised advice on how best to use the email app.

But the weakness of focus is tunnel vision. As Vohra explained some of the more niche features in Superhuman, one was particularly eye-catching: extremely detailed “read receipts” for emails, created using a “tracking pixel” silently embedded in the emails.

The email standard doesn’t allow for WhatsApp-style read-receipts, so instead companies that offer them – not just Superhuman but mostly powerful B2B services rather than consumer email clients – embed an invisible, transparent one-pixel image in every sent email, and track when that image is downloaded from their servers.

Superhuman does this too, but Vohra explained that the company’s implementation was characteristically more powerful than its competition: the pixel could distinguish different recipients opening the same email, differentiate mobile and desktop openings, and even report the approximate (state or country-level) location.

Vohra explained the positives of the approach, describing sending an email to an investor and holding off spamming them with follow-up missives after spotting that they were opening it from a mobile device in Hawaii – presumably on holiday.

But others spotted the negatives. “They’ve identified a feature that provides value to some of their customers (ie, seeing if someone has opened your email yet) and they’ve trampled the privacy of every single person they send email to in order to achieve that,” wrote fomer Twitter executive Mike Davidson, in a widely shared post. “Superhuman never asks the person on the other end if they are OK with sending a read receipt (complete with timestamp and geolocation). Superhuman never offers a way to opt out.”

Too much time thinking about what its hardworking power-users demand from email left the company blind to the wider ramifications of what it was making, Vohra now accepts. “I have come to understand that there are indeed nightmare scenarios involving location tracking. I should note that we deliberately do not show cities — we only show states or countries — but a determined attacker could still misuse this information,” he wrote in a chastened blogpost, the day after our last call.

“When we built Superhuman, we focused only on the needs of our customers. We did not consider potential bad actors. I wholeheartedly apologise for not thinking through this more fully.” The company no longer tracks location data at all, turns off the read receipts for all users by default, and is “prioritising” work to let Superhuman users opt out of loading images by default – the only practical way to protect against tracking pixels. (Tracking pixels are not limited to emails, and are common across the web. The Guardian’s website hosts one from Facebook, used to help advertise on the social network.)

The company has not removed the feature entirely, however. “We are still keeping the feature, as Superhuman is business software for email power users,” Vohra says. “In the ‘prosumer’ email market, read statuses have been ‘must have’ for many years … Before Superhuman had read statuses, it was both our most requested feature and also one of the most common reasons not to buy. In our market, the demand for read statuses is so high that it has now become table-stakes.” Vohra did not address another of Davidson’s suggestions: that the company disclose its use of tracking pixels to email recipients.

In a market where its main competitor is the vast surveillance machine of Google, Superhuman’s apology, and rapid decision to change its product, suggests there may be substance to its claim to be different. Still: feted by insiders, raising millions from the kings of venture capital, and plunging immediately into a crisis over personal data. Maybe Superhuman really is your archetypal Silicon Valley darling after all?