Superhuman, the $30/month email app that venture capitalists love, is being accused of enabling 'spying' on anyone who reads your messages

• Superhuman, an inbox management startup popular among venture investors, announced last week it raised $33 million in Series B funding led by Andreessen Horowitz. 
• The company has a cult-like following among Silicon Valley's biggest names and was valued at $260 million in Thursday's round, according to a New York Times report, and has raised at least $46 million according to Pitchbook data.
• On Sunday, Mike Davidson, founder of Newswire and former lead designer at Twitter, published a blog post taking Superhuman to task for using "tracking pixels" that let users see when, where, and how often recipients open their messages - often without the recipient's knowledge or permission. 
• Davidson's post set off a fierce debate: Some have said that the feature amounts to "surveillance" of the people reading your messages, while others counter that tracking pixels have been in use for a very long time, before Superhuman even existed.
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Over the last few years, the e-mail app Superhuman has proven very popular, especially among Silicon Valley investors and other power players - among Superhuman super-fans, the app's tools for managing your inbox are enough to offset the steep $30/month subscription fee that it's charging while in an invite-only beta phase.

Now, though, Superhuman is coming under fire for one particular feature: By default, it seems, messages sent via Superhuman contain a "tracking pixel" that let users see when, where, and how often the recipient opened an email sent through the service - allegedly, without telling that recipient.

The conversation was started with a blog post on Sunday by Mike Davidson, the founder of Newswire and former lead designer of Twitter, entitled "Superhuman is Spying on You," taking the company to task for not giving those who get such messages the option not to send that information back to the sender. 

"Superhuman never offers a way to opt out. Just as troublingly, Superhuman teaches its user to surveil by default," writes Davidson (emphasis his). "I imagine many users sign up for this, see the feature, and say to themselves "Cool! Read receipts! I guess that's one of the things my $30 a month buys me."

In the wake of Davidson's blog post, Superhuman updated its terms of service on Monday to include more detailed information on the read-reciept feature. However, Superhuman is technically still in that invite-only beta phase, meaning that only those invited users seem able to access those terms. 

Davidson's post set off heated debate between founders, investors, and other prominent figures in the tech industry over what data tracking is acceptable in a world where Facebook's Cambridge Analytica scandal have made issues of privacy and control over your data seem more pressing than ever.

Others say that this is a "nontroversy" - that Superhuman's technology is worth any privacy tradeoff, and that lots of products (especially email marketing products) use similar tracking tools. 

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The app has a cult-like following among founders and investors alike, with a mission t0 come up with a better way to handle e-mail -  interface that "needs to be reinvented," Santi Subotovsky of Emergence Capital told Business Insider in February. 

Last week, Superhuman announced it raised $33 million in Series B funding led by Andreessen Horowitz that valued the 5-year-old company at $260 million, according to a New York Times report. Based on Pitchbook data, the invite-only startup has raised at least $46 million since it was founded in 2014. 

Representatives from Superhuman and Andreessen Horowitz did not respond to Business Insider's request for comment.

Read Mike Davidson's full post here.