[Nutella4]

Here's an example: Conspiracy to commit a crime is a crime. Sometimes the conspiracy alone, that is just talking about breaking the law without ever doing it, is considered a crime.*

So an email suggesting something illegal with a reply agreeing can get you prosecuted and jailed. We all know how hard it is to understand tone from emails so even if you were joking you would have a hard time proving that in court. And if you weren't joking in the email but just imagining what might happen that can still be a prosecutable crime.

(*) https://en.wikipedia.org/wiki/United_States_v._Shabani