This was started by Dave Smyth over on the bird site:
Not to Pixels
Protest site for people to push back against tracking in most marketing emails - 99.999% of the time the senders have no consent for and is not permitted under PECR legislation in EU and UK.
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For context I think it was driven by this article which uses data from Hey by Basecamp
'Spy pixels in emails have become endemic'
No to pixels provides a template for complaining to the UK data protection regulator ICO, but could probably be adapted easily enough for a lot of other countries.