Back when Netscape had a mail client, it used a program called movemail
to transfer new incoming messages from the user's inbox (at that time, that would have been an mbox
spool I guess) to its own spool. I seem to recall that Emacs (RMAIL?) also followed this convention at some point. (In fact, now that I googled for a bit, Netscape apparently took this from Emacs, rather than the other way around.)
The "specification" was basically that Movemail would use some locking to make sure two processes would not attempt to fetch mail at the same time. As long as you always used the movemail
program to fetch mail, you would not need to worry about its internals.
Jamie Zawinski's page is probably the most well-researched document you can easily find, although it is hardly authoritative: http://www.jwz.org/doc/movemail.html
Here is (a random, possibly mirrored copy of) the Emacs program: ftp://www.ai.mit.edu/pub/mav/netscape/movemail-src/movemail.c
Update: (A much extended) movemail
is currently distributed as a part of GNU Mailutils