Now, what additional personal data are collected by tracking pixels?
> these are used to uniquely identify individuals
I would say that this isn't the case. It is to check that the email was read.
In theory, an IP can be captured, which is considered PII, but most mailbox providers use proxies so this isn't reliable.
You are correct that the email was already personal data. But, GDPR requires that each new use of data be transparently communicated and legally justified (which may or may not mean consent), even if it's only using data you already have. The fact that they have already identified the user does not resolve the issue--GDPR still cares when you collect more data about a known user.
Meaning, even though you are justified using the email address to send the newsletter, you may not be in the clear building an engagement profile associated with that email. Which, apparently, some email marketers do.
There's nothing stopping them either, they're entitled to do so, given they obtain consent for that data processing.
I'm yet to see a single marketer only do tracking by opt-in, and I work in the industry.
The problem is that we're:
A) Collecting the data without consent
B) In most cases, unable to not collect the data because ESP's do it by default with no option to switch it off
If I read it, when I read it, where I read it from (location, device, etc)
If linked to email address and considered personal data then the argument is what is covered by consent to receive email marketing? IMHO tracking whether the email was opened is covered (in the same way as agreeing to receive phone marketing should imply they can track whether you answered the phone...). They will also obviously keep track of what emails they sent you and when.
But agreeing to receive a snail mail does not imply consent to track if it was read. I feel that email is closer to that.
The point is that tracking emails, like tracking phone calls, is inherent to that communication medium and there is absolutely nothing wrong with it.