IEEE Access

Hard to See, Harder to Block: Exploring the Challenges in Email Tracking Pixels Detection

An evaluation of email tracking-pixel detection across 44,710 real-world emails from 620 sender domains and 22 commercial services, characterising how brittle deployed defenses are.

publishedFederico Cernera, Vincenzo Imperati, Massimo La Morgia, Alessandro Mei, Alberto Maria Mongardini, Eugenio Nerio Nemmi, Luciano Ortenzi, Francesco SassiDOI

Blocking tracking pixels in email is treated as a solved problem: mail clients and privacy tools ship with defenses on by default. This paper measures how well those defenses actually hold up against email as it is really sent.

Venue: IEEE Access
Type: Journal article (peer-reviewed)
Status: Published
Role: Measurement, data collection, analysis

Research question

How reliably do deployed tracking-pixel defenses detect tracking in real email, and what characteristics of a pixel cause them to fail?

Method

We assembled a corpus of real-world emails rather than synthetic samples, and evaluated detection across commercial tracking services and mail-side defenses, measuring not just whether a tracker was present, but whether each defense recognised it.

44,710

real-world emails

620

sender domains

commercial services

Findings

Detection is brittle in a specific way: defenses tend to recognise the canonical shapes of a tracking pixel and miss variants that are trivially different. Because the tracking services themselves vary in how they construct pixels, coverage differs sharply depending on which service sent the mail, which means a user’s effective protection depends on who is emailing them, not on the tool they chose.

Engineering contribution

The study needed a corpus that reflected real senders, and a harness that could run each defense against each message consistently. Building the collection and evaluation pipeline, with validation and reproducible runs, was the precondition for the measurement. I have written about the wider acquisition system it belongs to separately.

Limitations

The corpus covers the senders and services observed during the study window. Detection results describe the versions of the defenses evaluated at that time; both trackers and defenses change.

Cite

@article{Cernera_2025,
  title = {Hard to See, Harder to Block: Exploring the Challenges in Email Tracking Pixels Detection},
  volume = {13},
  ISSN = {2169-3536},
  url = {http://dx.doi.org/10.1109/ACCESS.2025.3628124},
  DOI = {10.1109/ACCESS.2025.3628124},
  journal = {IEEE Access},
  publisher = {Institute of Electrical and Electronics Engineers (IEEE)},
  author = {Cernera, Federico and Imperati, Vincenzo and La Morgia, Massimo and Mei, Alessandro and Maria Mongardini, Alberto and Nemmi, Eugenio Nerio and Ortenzi, Luciano and Sassi, Francesco},
  year = {2025},
  pages = {193186--193200}
}