mbox - file containing mail messages
The most common format for storage of mail messages is mbox
is a single file containing zero or more mail messages.
A message encoded in mbox
format begins with a From_
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@example.com Sat Jan 3 01:05:34 1996
Date: 3 Jan 1996 01:05:34 -0000
From: God <firstname.lastname@example.org>
To: email@example.com (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.
line always looks like From envsender date
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
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
HOW A MESSAGE IS DELIVERED¶
Here is how a program appends a message to an mbox
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
The program then copies the message, applying >From quoting
line. >From quoting
ensures that the resulting lines are not
lines: the program prepends a >
to any From_
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_
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
line or end of file, whichever comes first. It strips off
the final blank line and deletes the quoting of >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
format, popularized by Rahul Dhesi in June 1995.
The original mboxo
format quotes only From_
lines. As a result it is impossible to tell whether
From: firstname.lastname@example.org (D. J. Bernstein)
>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.
format is like mboxo
format, but includes a Content-Length
field with the number of bytes in the message. mboxcl2
format is like
but has no >From
quoting. These formats are used by
SVR4 mailers. mboxcl2
cannot be read safely by mboxrd
There are many locking mechanisms for mbox
always uses flock
on systems that have it, otherwise lockf
The delivery date in a From_
line does not specify a time zone.
always creates the delivery date in GMT so that mbox
files can be safely transported from one time zone to another.
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.