I would actually go as far as potentially considering email a distributed protocol for live-chat considering how quickly they are delivered [1]. I'm aware that is not how many people use them, but I believe that is more related to the interface and not the underlying technology. Somebody could probably built a live-chat messenger that looks like Teams or iMessage while actually just being an email client. Edit: This already exists [2].

[1] https://groups.io/email-provider-status [2] https://www.spikenow.com/features/conversational-email/