Apple - What format does Apple Mail store its emails in?
You can find a description of the format based on an reengineering effort here:
It has three parts:
- The length of part 2, in bytes
- The message itself
- Message metadata (XML Property List)
There was a question on Stack Overflow where the answer linked to a program to convert to mbox format.
It appears to be proprietary and therefore not well documented in the public domain.