Sadly, Midchuck is right...
The problem developed when software developers (MS in particular) extended the capabilities of their email applications to include sending & displaying "rich content" such as HTML.
HTML, in and of itself, would not be a problem either. The problem is that MS uses a version of the code behind Internet Explorer to display the email, which also interprets JavaScript (and automatically handles other sorts of active content.)
I remember using simple text-based email on Unix years ago, and the entire concept of and "email virus" was a standing joke. Just couldn't happen.
Amazingly, people still think I'm paranoid because I won't execute EXEs they send me as attachments, no matter how cute they are.
I suspect, however, that if Eudora had the market share that MS does, in attempts to sell more copies, they would eventually add "features" that compromised security. But maybe I'm wrong.

ihc