I'm running /e/os on my tablet and phone, the phone has Proton mail on it for 2 years. The tablet has zero trackers and the phone 3, both are the same ProtonMail account. Interesting is that the phone's listed trackers are from other apps that where allowed to access mail, like my watch app. I wonder if messages that where allowed to download blocked content are loading pixel trackers and e/os is seeing that as a tracker on the app but the real source is the message.

Hmmm, I havent noticed that before. I will have to check

I dont have an answer but perhaps relevant context: google analytics is by far the most used solution by marketeers, a lot of newsletters have code embedded so the sender can see if it the email is opened and if so what links are clicked. Opening an email is enough to pull the tracker though, since that is the action that makes the code start running. I would say it is much more likely the trackers are from the emails, but since i'm not a developers I don't know how much analytic software like this is used by them. Also I don't know how effective proton is at stripping them. You could consider using a forwarding service, but that can be a hassle too. I use addy.io, and also have good experience with duckduckgo's service. If you have a paid plan from proton i believe they offer this service too, but maybe it isn't more effective at blocking trackers than what you have now. They did acquire or merged with SimpleLogin, but don't know how much of their service is already implemented in the regular email service.