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 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_

     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 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 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 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 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 parallel, mbox files should generally not be
accessed without locking.

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

•    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.

•    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 system call to create a hard link
     named folder.lock to the temporary file. The success of
     the system call should be additionally verified using
     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 and 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

     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.

Thomas Roessler <roessler@does‐not‐exist.org>, Urs Janssen

The mbox format occurred in Version 6 AT&T Unix.
A variant of this format was documented in RFC976.