Ubuntu Manpage: mbox - Format for mail message storage.

Provided by: mutt_1.9.4-3ubuntu0.6_amd64

NAME

       mbox - Format for mail message storage.

DESCRIPTION

This document describes the format traditionally used by Unix hosts to store mail messages locally. mbox
files typically reside in the system's mail spool, under various names in users' Mail directories, and
under the name mbox in users' home directories.

An 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 RFC822, RFC2822. The file format is line-
oriented. Lines are separated by line feed characters (ASCII 10).

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.

The sender address is expected to be addr-spec as defined in RFC2822 3.4.1. The date is expected to be
date-time as output by 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.

Example:

>From example@example.com Fri Jun 23 02:56:55 2000

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.

There are two different quoting schemes, the first (MBOXO) only quotes plain "From " lines in the body by
prepending a '>' to the line; the second (MBOXRD) also quotes already quoted "From " lines by prepending
a '>' (i.e. ">From ", ">>From ", ...). The later has the advantage that lines like

>From the command line you can use the '-p' option

aren't dequoted wrongly as a MBOXRD-MDA would turn the line into

>>From the command line you can use the '-p' option

before storing it. Besides MBOXO and MBOXRD there is also MBOXCL which is MBOXO with a "Content-
Length:"-field with the number of bytes in the message body; some MUAs (like mutt(1)) do automatically
transform MBOXO mailboxes into MBOXCL ones when ever they write them back as MBOXCL can be read by any
MBOXO-MUA without any problems.

If the modification-time (usually determined via stat(2)) of a nonempty 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.

LOCKING

Since mbox files are frequently accessed by multiple programs in parallel, mbox files should generally
not be accessed without locking.

Three different locking mechanisms (and combinations thereof) are in general use:

• fcntl(2) locking is mostly used on recent, POSIX-compliant systems. Use of this locking method is,
in particular, advisable if mbox files are accessed through the Network File System (NFS), since
it seems the only way to reliably invalidate NFS clients' caches.

• flock(2) locking is mostly used on BSD-based systems.

• Dotlocking is used on all kinds of systems. In order to lock an mbox file named folder, an
application first creates a temporary file with a unique name in the directory in which the folder
resides. The application then tries to use the link(2) system call to create a hard link named
folder.lock to the temporary file. The success of the link(2) system call should be additionally
verified using stat(2) calls. If the link has succeeded, the mail folder is considered dotlocked.
The temporary file can then safely be unlinked.

In order to release the lock, an application just unlinks the folder.lock file.

If multiple methods are combined, implementors should make sure to use the non-blocking variants of the
fcntl(2) and flock(2) system calls in order to avoid deadlocks.

If multiple methods are combined, an 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.

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 mbox files. Failure to do so may result in
loss of e-mail data, and in corrupted mbox files.

FILES

       /var/spool/mail/$LOGNAME
              $LOGNAME's incoming mail folder.

       $HOME/mbox
              user's archived mail messages, in his $HOME directory.

       $HOME/Mail/
              A directory in user's $HOME directory which is commonly used to hold mbox format folders.

AUTHOR

       Thomas Roessler <roessler@does-not-exist.org>, Urs Janssen <urs@tin.org>

HISTORY