mbox - Format for mail message storage.
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
in users' home directories.
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
. 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_
The sender address is expected to be addr-spec
as defined in
3.4.1. The date is expected to be date-time
as output by
. 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.
- >From firstname.lastname@example.org 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'
before storing it. Besides MBOXO
there is also
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
ever they write them back as MBOXCL
can be read by any MBOXO
without any problems.
If the modification-time (usually determined via stat(2)
) of a nonempty
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.
files are frequently accessed by multiple programs in
files should generally not be accessed without locking.
Three different locking mechanisms (and combinations thereof) are in general
- 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
- flock(2) locking is mostly used on BSD-based
- 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)
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
$LOGNAME's incoming mail folder.
user's archived mail messages, in his
A directory in user's $HOME directory
which is commonly used to hold mbox format folders.
Thomas Roessler <email@example.com>, Urs Janssen
format occurred in Version 6 AT&T Unix.
A variant of this format was documented in RFC976