You surf the website and you see this beautiful, colorful, lively email program. You are attracted and impressed and you end up buying it. But when you install and run it, it bears no resemblance to the photos you saw on the website. Because those were of the OS X / macOS version of the program. You feel deceived, especially because the Windows version looks so boring.
This happened to me. I felt disappointed with the looks and the feel of Postbox after having bought and installed it. I felt even more bad when I realized that, coincidentally [I didn't know this before], the Mozilla Thunderbird that I uninstalled in order to switch to Postbox is the same Thunderbird on which Postbox itself is based. Such a bad feeling - Postbox website never tells you about this.