The entire point of an AFJ is that they don't know it's a joke. As the name says, the goal is to "fool" them. They don't know it's a "silly April Fool's email".

Designing an AFJ is tricky, and the larger your audience the trickier it gets. Your friends know you're a jokester; they figure it out almost immediately. When you send it out to a bunch of people you don't know, somebody is going to forget the date and assume you're serious -- because it's supposed to look serious.

Further, if it looks like something that might be a problem they have to solve, somebody is going to start solving that problem immediately. You don't know what's going on in their day -- if they've already got six crises going, they're not going to "take a joke" well.

The wider your audience, the more obvious you have to be. Knowing how to deliver a joke is also an important life skill -- as is learning not to blame your failure on the target.