It's been over 20 years, but I used to build batch processing pipelines using SMTP (not Outlook). Biggest "choose your own adventure" aspect is accounting / auditing (making sure you didn't lose batches). I still joke about it as an option; although if somebody took me up on it I'd write up at least a semi-serious proposal.

In the middle are Mule, Rabbit, Kafka, ZMQ.

At the other end is UDP multicast, and I still use that for VM hosts or where I can trust the switch.