We already talk about why HTML emails are bad, but that is the default in most of the email service providers. HTML emails means some code is getting executed and rendered on your system. Maybe on a browser, or on a desktop email client.
Many people do not use any HTML tag in their emails, but then they have fancy email signatures. A lot of time they have fancy image generated on a website and they use the generated image URL as signature. This means every time someone opened the email (with HTML rendering on) the third party company will be able to track those usages. We don't know what happens next to all of these tracking information.
Last week I was trying out various desktop email clients available on Fedora 32, and noticed a strange thing on Kmail/Kontact, the email client of KDE. I run my Unoon tool to monitor all processes for any network connection on system. And, suddenly it popped a notification about Kmail connecting to mysignatures.io. I was surprised for a second, as Kmail also disables loading of any remote resource (say images) and does not render HTML email by default.
Then I figured that if I click on reply button (the compose window), it fetches
the image from the signature (or any
<img> tag). This means the HTML is
getting rendered somehow, even if it is not showing to the user. After I filed
a bug upstream, I also pinged my
helped to reproduce it and also find more details on the same. Now, we are
waiting for a fix. I hope this does not involve JS execution during that
I also checked for same behavior in Thunderbid, and it does not render in similar way.