zdnet[.]com/article/inbox-promo-emails-tracking-you-proton-mail-warns/
Retailers, along with the bad folks sending spam and phishing attempts, add trackers to every email they send.
These small, invisible images, as well as tracker links, are able to log information on our location, the time an email is opened, and our device type, all of which help retailers build a profile on us and potentially our shopping habits.As Proton Mail calls it, these practices "turn the simple act of opening an email into a data collection event."
When you open an email it also verifies that it's a good email address, so you know you'll get repeat emails if/when you open one.