system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How can merely opening an email be dangerous, if you don't download the attachments?
gpt
Email at first was text only, now it can contain images, gifs, etc. This is because web technologies (HTML, CSS) spilled into email territory.
So, the email clients (Outlook, Thunderbird, etc...) now need the ability to render emails like a web browser but instead of reinventing the wheel and create a render engine, is easier to reuse an existing engine from a web browser to do so.
This open the door to vulnerabilities that can affect you with only looking at an email the same way a web page does, because now the email client has to interpret the code in the email to render it, and wherever code has to be interpreted there's always the danger of an exploit lurking.