"Digest" mode typically means you get a single e-mail every day or so with all the messages sent around since the last digest. "Delivery" means you get every message, individually, as soon as the server can forward it.
There's also a carbon-dev list, which I'm pretty sure is public. Huge traffic. I had to switch to digest mode. Apple engineers and other knowledgeable folk. Occasional talk about general architectural/OSX issues, not strictly limited to Carbon.