Yep, Proton Blocks the Pixels

Did you know? Protonmail blocks most open tracking pixels.

Proton Mail is a privacy-focused, encrypted email service intended to protect users' communications from surveillance and unauthorized access. The company Proton AG was founded in 2013 by scientists from CERN and is headquartered in Switzerland.

Part of that privacy-focused mindset includes proactively blocking certain types of tracking. This includes open tracking or open detection, the tiny pixels used by email senders, primarily email marketing senders, to denote whether or not an email message has been read by a recipient. Bulk email senders typically track opens using an invisible image URL embedded in the HTML email body of an email message. The assumption being made is that when/if this image is loaded, it is because the email recipient has chosen to view a given email message, and thus, the email client (MUA) or webmail interface displays and renders the HTML body content, including loading any externally linked images. The open tracking pixel is one of those images.

Once a cornerstone metric of email marketing success, email open tracking has become less accurate over time. “Open tracking is nothing but bots talking to bots,” one mailbox provider’s product manager is keen to tell me every time the topic comes up. What he means is that in this modern age, Apple, Google, Yahoo and others all utilize proxy mechanisms (like, famously, Apple’s Mail Privacy Protection).

If you want to learn more about Apple Mail Privacy Protection, I’ve got a roundup here, and I even co-presented a webinar on Apple MPP a couple of years ago.

Though proxying and privacy protection limits the accuracy of open tracking today, many marketers still utilize the metric to measure email marketing success, and often express concern if one or more domains are out of sync from most others, perhaps with zero opens tracked, due to the various new privacy-oriented restricting functionality implemented in various places.

Indeed, various marketers have noted that their sends to Proton Mail subscribers, while likely getting delivered to the inbox, were not logging any opens whatsoever. That is indeed because of Proton's intent to block open tracking pixels. I don't know how they've chosen to implement the blocking; it is possible that it's a fixed list of known hostnames, domains, URL strings or folder paths. It may not block all, and additional tracking pixels, if not blocked today, could be blocked tomorrow.

For those senders wondering about scope -- how many subscribers does this particular blocking affect? Proton is not the world’s largest mailbox provider, but they do host mail for 8,931 domains (in the top ten million) as of February 2025. Their primary user email domains are proton.me, protonmail.com, pm.me, and protonmail.ch.