You should be aware by now that every tracking mechanism is met with various countermeasures by the mailclients and servers to guaranty privacy.

One of the reasons are spammers who need feedback about their success.

For instance a server might decide to pre-fetch all images before the mail is read and include them as inlined base 64.

AFAIR is Gmail doing this.

Updates

• https://www.theverge.com/2019/7/3/20681508/tracking-pixel-email-spying-superhuman-web-beacon-open-tracking-read-receipts-location
• https://www.macrumors.com/2021/06/10/ios-15-mail-privacy-protection-tracking-pixels/