Every program attempts to expand until it can read mail. Those programs which cannot so expand are replaced by ones which can.
The law is also known as Zawinski's Law of Software Envelopment. Last December, Zoom announced plans to expand to new products – including possibly an email service.
Even tools that are the best examples of the Unix philosophy, do one thing and do it well like curl are victim to Zawinksi's law:
It's not so much about email as it is about every software's broader desire to be a platform.
In the last decade, the trend was more evident in the opposite direction – messaging software trying to do everything else. Look no further than the proliferation of features in Slack or Microsoft Teams. Of course, those systems still send you lots of emails.
What is today's version of Zawinski's law, i.e., what is the feature that makes products more like a platform?
It could be the trend of embedded finance – SaaS tools that offer financial services like bank accounts or working capital to their customers, making those products into vertical platforms.
It could be collaborative features like video chat, multiplayer data-structures for real-time editing, or just chat. Or maybe it's a plugin system that drastically extends the functionality of an application to satisfy the long-tail or use cases.
Fun fact: Zawinski was one of the early employees at Netscape. He's the one who is given credit for convincing Netscape to open source its browser under Mozilla (a name he also came up with). And not to escape his own law, but he worked on Netscape Mail, the first mail reader that supported HTML.