Is there any reason why "/var/mail/$user" (usually in trad. UNIX format) cannot itself be in "mbx" format? (Trying it as such seems not to work, and this seems to be confirmed by the "docs/drivers.txt" description.)
That's a good question! Fortunately, there are good answers:
The spool directory is perceived by a large body of software to be "owned" by the traditional UNIX mail facility. As such, there is all sorts of software which will happily append a traditional UNIX format message to a file in the spool directory without any check to see if, in fact, that file actually is a traditional UNIX format message. The result is a corrupted mbx format mailbox.
You would think "shouldn't programs invoke sendmail, or at least pipe to /bin/mail or /bin/mail.local or whatever?" Of course! But all too many programs write directly because "it is more efficient that way."
Another technical issue is that using the spool directory for any other format breaks the capability of a user independently deciding to use another format. Most software packages convert from traditional UNIX format to another format; arbitrary format conversion is clanky at best.
As for philosophical questions:
There are system management and privacy concerns for not having people's INBOX in a shared spool directory.
It is easier to administer quotas when the INBOX is co-located with the non-INBOX mailboxes. It is perfectly reasonable to set the user's "mailbox directory" to be something other than the user's UNIX home directory, and many sites do this.
Anyone can obtain information about the size, last write, and last read of anyone else's INBOX in a shared directory. This can be, in certain circumstances, extremely useful for bad guys.
-- Mark --
http://staff.washington.edu/mrc Science does not emerge from voting, party politics, or public debate. Si vis pacem, para bellum.
