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.\" -*-nroff-*-
.\"
.\" Copyright (C) 2000 Thomas Roessler <roessler@does-not-exist.org>
.\"
.\" This document is in the public domain and may be distributed and
.\" changed arbitrarily.
.\"
.TH mbox 5 "2002-02-19" Unix "User Manuals"
.\"
.SH NAME
mbox
\-
Format for mail message storage.
.\"
.SH DESCRIPTION
This document describes the format traditionally used by Unix hosts
to store mail messages locally.
.B mbox
files typically reside in the system's mail spool,
under various names in users' Mail directories,
and under the name
.B mbox
in users' home directories.
.PP
An
.B mbox
is a text file containing an arbitrary number of e-mail messages.
Each message consists of a postmark,
followed by an e-mail message
formatted according to \fBRFC822\fP, \fBRFC2822\fP.
The file format is line-oriented.
Lines are separated by line feed characters (ASCII 10).
.PP
A postmark line consists of the four characters "From",
followed by a space character,
followed by the message's envelope sender address,
followed by whitespace,
and followed by a time stamp.
This line is often called From_ line.
.PP
The sender address is expected to be
.B addr-spec
as defined in \fBRFC2822\fP 3.4.1.
The date is expected to be
.B date-time
as output by
.BR asctime (3).
For compatibility reasons with legacy software,
two-digit years greater than or equal to 70
should be interpreted as the years 1970+,
while two-digit years less than 70
should be interpreted as the years 2000-2069.
Software reading files in this format
should also be prepared to accept non-numeric timezone information
such as "CET DST" for Central European Time, daylight saving time.
.PP
Example:
.IP "" 1
>From example@example.com Fri Jun 23 02:56:55 2000
.PP
In order to avoid misinterpretation of lines in
message bodies which begin with the four characters "From",
followed by a space character,
the mail delivery agent must quote any occurrence of
"From " at the start of a body line.
.sp
There are two different quoting schemes,
the first (\fBMBOXO\fP) only quotes plain "From " lines in the body
by prepending a '>' to the line;
the second (\fBMBOXRD\fP) also quotes already quoted "From " lines
by prepending a '>' (i.e. ">From ", ">>From ", ...).
The later has the advantage that lines like
.IP "" 1
>From the command line you can use the '\-p' option
.PP
aren't dequoted wrongly as a \fBMBOXRD\fP-MDA would turn the line into
.IP "" 1
>>From the command line you can use the '\-p' option
.PP
before storing it.
Besides \fBMBOXO\fP and \fBMBOXRD\fP there is also \fBMBOXCL\fP
which is \fBMBOXO\fP with a "Content-Length:" field
with the number of bytes in the message body;
some MUAs (like
.BR neomutt (1))
do automatically transform \fBMBOXO\fP mailboxes into \fBMBOXCL\fP ones
when ever they write them back as \fBMBOXCL\fP
can be read by any \fBMBOXO\fP-MUA
without any problems.
.PP
If the modification-time
(usually determined via
.BR stat (2))
of a nonempty
.B mbox
file is greater than the access-time the file has new mail.
Many MUAs place a Status: header in each message
to indicate which messages have already been read.
.\"
.SH LOCKING
Since
.B mbox
files are frequently accessed by multiple programs in parallel,
.B mbox
files should generally not be accessed without locking.
.PP
Three different locking mechanisms (and combinations thereof)
are in general use:
.IP "\(bu"
.BR fcntl (2)
locking is mostly used on recent,
POSIX-compliant systems.
Use of this locking method is, in particular, advisable if
.B mbox
files are accessed through the Network File System (NFS),
since it seems the only way to reliably invalidate NFS clients' caches.
.IP "\(bu"
.BR flock (2)
locking is mostly used on BSD-based systems.
.PP
If multiple methods are combined,
implementers should make sure to
use the non-blocking variants of the
.BR fcntl (2)
and
.BR flock (2)
system calls in order to avoid deadlocks.
.PP
If multiple methods are combined,
an
.B mbox
file must not be considered to have been successfully locked
before all individual locks were obtained.
When one of the individual locking methods fails,
an application should release all locks it acquired successfully,
and restart the entire locking procedure from the beginning,
after a suitable delay.
.PP
The locking mechanism used on a particular system is a matter of local policy,
and should be consistently used by all applications
installed on the system which access
.B mbox
files.
Failure to do so may result in loss of e-mail data,
and in corrupted
.B mbox
files.
.\"
.SH FILES
.IR /var/spool/mail/ $LOGNAME
.RS
\fB$LOGNAME\fP's incoming mail folder.
.RE
.PP
.RI $HOME /mbox
.RS
user's archived mail messages,
in his \fB$HOME\fP directory.
.RE
.PP
.RI $HOME /Mail/
.RS
A directory in user's \fB$HOME\fP directory which is commonly used to hold
.B mbox
format folders.
.RE
.\"
.SH "SEE ALSO"
.BR neomutt (1),
.BR fcntl (2),
.BR flock (2),
.BR link (2),
.BR stat (2),
.BR asctime (3),
.BR maildir (5),
.BR mmdf (5),
.BR RFC822 ,
.BR RFC976 ,
.B RFC2822
.\"
.SH AUTHOR
Thomas Roessler <roessler@does-not-exist.org>, Urs Janssen <urs@tin.org>
.\"
.SH HISTORY
The
.B mbox
format occurred in Version 6 AT&T Unix.
.br
A variant of this format was documented in \fBRFC976\fP.
|
