mbox(5) User Manuals mbox(5)
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 sys-
tem'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 mes-
sages. Each message consists of a postmark, followed by an e-mail mes-
sage formatted according to RFC5322. 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 of-
ten called From_ line.
The sender address is expected to be addr-spec as defined in RFC5322
3.4.1. The date is expected to be date-time as defined in RFC5322 3.3.
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 pre-
pared to accept non-numeric timezone information such as "CET DST" for
Central European Time, daylight saving time.
From email@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
>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.
Since mbox files are frequently accessed by multiple programs in paral-
lel, mbox files should generally not be accessed without locking.
Three different locking mechanisms (and combinations thereof) are in
o fcntl(2) locking is mostly used on recent, POSIX-compliant sys-
tems. 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'
o flock(2) locking is mostly used on BSD-based systems.
o 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 re-
sides. 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
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 ob-
tained. When one of the individual locking methods fails, an applica-
tion 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.
$LOGNAME's incoming mail folder.
user's archived mail messages, in his $HOME directory.
A directory in user's $HOME directory which is commonly used to
hold mbox format folders.
mutt(1), fcntl(2), flock(2), link(2), stat(2), maildir(5), mmdf(5),
RFC976, RFC4155, RFC5322
Thomas Roessler <firstname.lastname@example.org>, Urs Janssen
The mbox format occurred in Version 6 AT&T Unix.
A variant of this format was documented in RFC976 and RFC4155.
Unix November 4th, 2013 mbox(5)