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(emacs)Rmail Inbox

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Rmail Files and Inboxes
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   The operating system places incoming mail for you in a file that we
call your "inbox".  When you start up Rmail, it runs a C program called
`movemail' to copy the new messages from your inbox into your primary
Rmail file, which also contains other messages saved from previous
Rmail sessions.  It is in this file that you actually read the mail
with Rmail.  This operation is called "getting new mail".  You can get
new mail at any time in Rmail by typing `g'.

   The variable `rmail-primary-inbox-list' contains a list of the files
which are inboxes for your primary Rmail file.  If you don't set this
variable explicitly, it is initialized from the `MAIL' environment
variable, or, as a last resort, set to `nil', which means to use the
default inbox.  The default inbox is `/var/mail/USERNAME',
`/usr/spool/mail/USERNAME', or `/usr/mail/USERNAME', depending on your
operating system.

   To see what the default is on your system, use `C-h v
rmail-primary-inbox <RET>'.  You can specify the inbox file(s) for any
Rmail file with the command `set-rmail-inbox-list'; see Note: Rmail
Files.

   There are two reasons for having separate Rmail files and inboxes.

  1. The inbox file format varies between operating systems and
     according to the other mail software in use.  Only one part of
     Rmail needs to know about the alternatives, and it need only
     understand how to convert all of them to Rmail's own format.

  2. It is very cumbersome to access an inbox file without danger of
     losing mail, because it is necessary to interlock with mail
     delivery.  Moreover, different operating systems use different
     interlocking techniques.  The strategy of moving mail out of the
     inbox once and for all into a separate Rmail file avoids the need
     for interlocking in all the rest of Rmail, since only Rmail
     operates on the Rmail file.

   Rmail was written to use Babyl format as its internal format.  Since
then, we have recognized that the usual inbox format on Unix and GNU
systems is adequate for the job, and we plan to change Rmail to use that
as its internal format.  However, the Rmail file will still be separate
from the inbox file, even on systems where their format is the same.