Speaking of mbox, an oldie-but-goodie from JWZ on the mbox format specification, or lack thereof:
RFCs specify internet protocols, that is, on-the-wire formats. The
thing that the original poster is looking for is a description of the
BSD Mailbox file format (which is not something an RFC would cover.)
But, here’s the good news, there is no true specification of this file
format, just a collection of word-of-mouth behaviors of the various
programs over the last few decades which have used that format.