It always was a message queue in a very literal sense.

There's a lot of work in mailer-daemons to ensure that email has as reliable as possible delivery in a store-and-forward system..

You're correct - I left out the word "universal" accidentally, which would have made my intent much more clear.

Thanks for catching that.

I think this is what a lot of people who complain about Slack don't get. It's just a better message queue for your business. The fact that you can funnel all your business events, regardless of whether they originate from humans or bots, into one place and then each worker (again, either human or bot) can subscribe/filter/react to relevant events is super powerful. However, if you try to use it as a corporate SMS platform or email replacement, you will very quickly feel overwhelmed because both of those message queues are designed for much lower throughput.

And you can literally use the maildir format for a queue!

https://pypi.org/project/dirq/

Perl had the original implementation, and there are implementations in other languages.

I forgot about that - used to be a cool hack!