> What bothers me (and I accept this may be a purely theoretical
objection) is the potential for effectively merging and mixing of
disparate mbox files.
it's been going on for years, due to NFS-mounted mail spool directories
being written to by /bin/mail (and other) delivery programs and POP
servers and user agents on various platforms. not only were there
slight differences in mbox format, there were several different and
incompatible styles of locking. and yes, it caused problems. defining a
MIME label for mbox isn't going to change any of this.
now if I were to write a recommendation for how to write mbox files,
I'd say that all body parts MUST be encoded if necessary to prevent
the characters "From" from appearing at the beginning of a line.
I'd also say that Content-Length MUST NOT be written, that From_ dates
have to be in exactly such-and-such a format, that Return-Path fields
are REQUIRED when storing a message received by SMTP, even if they're
redundant with From_ lines, etc. but that's not the problem we're
currently trying to solve.