Yes/No (or tick/cross, thumbs up/down etc.) are rarely intuitive by themselves, they need context unambiguously defined. As it says in the original article – even when you know the context, you can still have a nagging doubt if there is space for ambiguity, and that’s about as unintuitive as you can get.

A good icon for that in my humble opinion would be an envelope behind bars! haha. Possibly a halo or wings over the positive mail. But not horns for negative because calling something devilish can actually be construed as a good thing too.

Looking at it, it doesn’t make much sense to me. Is that supposed to be a “spy”?

I like your spam/not spam icons.

While the choice of icons is definitely not a good one, keep in mind that most of the people who design plug-ins are developers not designers. Many of them have probably never really thought of bringing a designer on board either. These people are usually focused on what they are developing and give the design aspect of it little if any thought.

So how do we go about improving the designs of free/open-source software? Maybe as designers, we should volunteer our time, as these developers have, and help them create better designs for the UIs they are developing?

BTW, I personally think that as far as the aforementioned icons go, the ubiquitous red circle with a line through it, and nothing else accompanying it, would be a good choice for the Spam icon (its something you don’t want), while a lone green check mark would be a good choice for the mail you want to keep. In my mind, icons should be as simple as possible while conveying as much as possible. Given the context, an email client and the associated spam plug-in, I believe that would be pretty intuitive.

Ideally I wouldn’t want to risk confusing mail I never want to see (spam) with good mail I’m just cleaning up.