TBH, I've never understood why more people didn't explore the multipart content type feature of email to extend email. It's typical to have plaintext and html in the same email, but why no JSON using a well defined and published JSON-schema.
Is it to bypass firewalls in some way ?
It worked pretty well.
The nice thing was all the existing tools that works well with various aspects of e-mail, and the great ease of introspection and testing.
The second time I did it was at Edgeio, where I initially used it to get a prototype of our blog feed retrieval / indexing pipeline up and running quickly. Though we relatively soon replaced it with a homegrown Stomp server, mostly because a lot of the stuff we were doing didn't need the delivery guarantees of e-mail, so we used in-memory queues for a lot of stuff.
It works great for some workloads, and it's a very easy way of prototyping things that makes it easy to debug message flows etc.. The state of open source message brokers have drastically improved since the times I did it, though - the first time we did it was back in 2002 or 2003.