It was bad even 20 years ago, I remember a discussion when we were trying to implement something coining the phrase "University of Washington brain damage". It seems to have been extremely tightly tied to the UW-IMAP implementations.

The need for a better protocol is why JMAP exists: https://jmap.io/spec-core.html

Although I see their point about non-REST, I think there's still a case for having a REST flavour available even if it may not be the most efficient.

I wonder if anyone's tried mail delivery over git repo yet.