mbox(5) Headers, Tables, and Macros mbox(5)
NAME
mbox - file containing mail messages
INTRODUCTION
The most common format for storage of mail messages is mbox
format. An mbox is a single file containing zero or more
mail messages.
MESSAGE FORMAT
A message encoded in mbox format begins with a From_ line,
continues with a series of non-From_ lines, and ends with a
blank line. A From_ line means any line that begins with
the characters F, r, o, m, space:
From [email protected] Sat Jan 3 01:05:34 1996
Return-Path: <[email protected]>
Delivered-To: [email protected]
Date: 3 Jan 1996 01:05:34 -0000
From: God <[email protected]>
To: [email protected] (D. J. Bernstein)
How's that mail system project coming along?
The final line is a completely blank line (no spaces or
tabs). Notice that blank lines may also appear elsewhere in
the message.
The From_ line always looks like From envsender date
moreinfo. envsender is one word, without spaces or tabs; it
is usually the envelope sender of the message. date is the
delivery date of the message. It always contains exactly 24
characters in asctime format. moreinfo is optional; it may
contain arbitrary information.
Between the From_ line and the blank line is a message in
RFC 822 format, as described in qmail-header(5), subject to
>From quoting as described below.
HOW A MESSAGE IS DELIVERED
Here is how a program appends a message to an mbox file.
It first creates a From_ line given the message's envelope
sender and the current date. If the envelope sender is
empty (i.e., if this is a bounce message), the program uses
MAILER-DAEMON instead. If the envelope sender contains
spaces, tabs, or newlines, the program replaces them with
hyphens.
The program then copies the message, applying >From quoting
to each line. >From quoting ensures that the resulting
lines are not From_ lines: the program prepends a > to any
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From_ line, >From_ line, >>From_ line, >>>From_ line, etc.
Finally the program appends a blank line to the message. If
the last line of the message was a partial line, it writes
two newlines; otherwise it writes one.
HOW A MESSAGE IS READ
A reader scans through an mbox file looking for From_ lines.
Any From_ line marks the beginning of a message. The reader
should not attempt to take advantage of the fact that every
From_ line (past the beginning of the file) is preceded by a
blank line.
Once the reader finds a message, it extracts a (possibly
corrupted) envelope sender and delivery date out of the
From_ line. It then reads until the next From_ line or end
of file, whichever comes first. It strips off the final
blank line and deletes the quoting of >From_ lines and
>>From_ lines and so on. The result is an RFC 822 message.
COMMON MBOX VARIANTS
There are many variants of mbox format. The variant
described above is mboxrd format, popularized by Rahul Dhesi
in June 1995.
The original mboxo format quotes only From_ lines, not
>From_ lines. As a result it is impossible to tell whether
From: [email protected] (D. J. Bernstein)
To: [email protected]
>From now through August I'll be doing beta testing.
Thanks for your interest.
was quoted in the original message. An mboxrd reader will
always strip off the quoting.
mboxcl format is like mboxo format, but includes a Content-
Length field with the number of bytes in the message.
mboxcl2 format is like mboxcl but has no >From quoting.
These formats are used by SVR4 mailers. mboxcl2 cannot be
read safely by mboxrd readers.
UNSPECIFIED DETAILS
There are many locking mechanisms for mbox files. qmail-
local always uses flock on systems that have it, otherwise
lockf.
The delivery date in a From_ line does not specify a time
zone. qmail-local always creates the delivery date in GMT
so that mbox files can be safely transported from one time
zone to another.
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If the mtime on a nonempty mbox file is greater than the
atime, the file has new mail. If the mtime is smaller than
the atime, the new mail has been read. If the atime equals
the mtime, there is no way to tell whether the file has new
mail, since qmail-local takes much less than a second to
run. One solution is for a mail reader to artificially set
the atime to the mtime plus 1. Then the file has new mail
if and only if the atime is less than or equal to the mtime.
Some mail readers place Status fields in each message to
indicate which messages have been read.
SEE ALSO
maildir(5), qmail-header(5), qmail-local(8)
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