1mbox(5) User Manuals mbox(5)
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6 mbox - Format for mail message storage.
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9 This document describes the format traditionally used by Unix hosts to
10 store mail messages locally. mbox files typically reside in the sys‐
11 tem's mail spool, under various names in users' Mail directories, and
12 under the name mbox in users' home directories.
13 An mbox is a text file containing an arbitrary number of e-mail mes‐
15 sages. Each message consists of a postmark, followed by an e-mail mes‐
16 sage formatted according to RFC5322. The file format is line-oriented.
17 Lines are separated by line feed characters (ASCII 10).
18 A postmark line consists of the four characters "From", followed by a
20 space character, followed by the message's envelope sender address,
21 followed by whitespace, and followed by a time stamp. This line is
22 often called From_ line.
23 The sender address is expected to be addr-spec as defined in RFC5322
25 3.4.1. The date is expected to be date-time as defined in RFC5322 3.3.
26 For compatibility reasons with legacy software, two-digit years greater
27 than or equal to 70 should be interpreted as the years 1970+, while
28 two-digit years less than 70 should be interpreted as the years
29 2000-2069. Software reading files in this format should also be pre‐
30 pared to accept non-numeric timezone information such as "CET DST" for
31 Central European Time, daylight saving time.
32 Example:
34 From example@example.com Fri Jun 23 02:56:55 2000
36 In order to avoid misinterpretation of lines in message bodies which
38 begin with the four characters "From", followed by a space character,
39 the mail delivery agent must quote any occurrence of "From " at the
40 start of a body line.
41 There are two different quoting schemes, the first (MBOXO) only quotes
43 plain "From " lines in the body by prepending a '>' to the line; the
44 second (MBOXRD) also quotes already quoted "From " lines by prepending
45 a '>' (i.e. ">From ", ">>From ", ...). The later has the advantage that
46 lines like
47 >From the command line you can use the '-p' option
49 aren't dequoted wrongly as a MBOXRD-MDA would turn the line into
51 >>From the command line you can use the '-p' option
53 before storing it. Besides MBOXO and MBOXRD there is also MBOXCL which
55 is MBOXO with a "Content-Length:"-field with the number of bytes in the
56 message body; some MUAs (like mutt(1)) do automatically transform MBOXO
57 mailboxes into MBOXCL ones when ever they write them back as MBOXCL can
58 be read by any MBOXO-MUA without any problems.
59 If the modification-time (usually determined via stat(2)) of a nonempty
61 mbox file is greater than the access-time the file has new mail. Many
62 MUAs place a Status: header in each message to indicate which messages
63 have already been read.
64
66 Since mbox files are frequently accessed by multiple programs in paral‐
67 lel, mbox files should generally not be accessed without locking.
68 Three different locking mechanisms (and combinations thereof) are in
70 general use:
71 · fcntl(2) locking is mostly used on recent, POSIX-compliant sys‐
73 tems. Use of this locking method is, in particular, advisable if
74 mbox files are accessed through the Network File System (NFS),
75 since it seems the only way to reliably invalidate NFS clients'
76 caches.
77 · flock(2) locking is mostly used on BSD-based systems.
79 · Dotlocking is used on all kinds of systems. In order to lock an
81 mbox file named folder, an application first creates a temporary
82 file with a unique name in the directory in which the folder
83 resides. The application then tries to use the link(2) system
84 call to create a hard link named folder.lock to the temporary
85 file. The success of the link(2) system call should be addition‐
86 ally verified using stat(2) calls. If the link has succeeded,
87 the mail folder is considered dotlocked. The temporary file can
88 then safely be unlinked.
89 In order to release the lock, an application just unlinks the
91 folder.lock file.
92 If multiple methods are combined, implementors should make sure to use
94 the non-blocking variants of the fcntl(2) and flock(2) system calls in
95 order to avoid deadlocks.
96 If multiple methods are combined, an mbox file must not be considered
98 to have been successfully locked before all individual locks were
99 obtained. When one of the individual locking methods fails, an applica‐
100 tion should release all locks it acquired successfully, and restart the
101 entire locking procedure from the beginning, after a suitable delay.
102 The locking mechanism used on a particular system is a matter of local
104 policy, and should be consistently used by all applications installed
105 on the system which access mbox files. Failure to do so may result in
106 loss of e-mail data, and in corrupted mbox files.
107
109 /var/spool/mail/$LOGNAME
110 $LOGNAME's incoming mail folder.
111 $HOME/mbox
113 user's archived mail messages, in his $HOME directory.
114 $HOME/Mail/
116 A directory in user's $HOME directory which is commonly used to
117 hold mbox format folders.
118
120 mutt(1), fcntl(2), flock(2), link(2), stat(2), maildir(5), mmdf(5),
121 RFC976, RFC4155, RFC5322
122
124 Thomas Roessler <roessler@does-not-exist.org>, Urs Janssen
125 <urs@tin.org>
126
128 The mbox format occurred in Version 6 AT&T Unix.
129 A variant of this format was documented in RFC976 and RFC4155.
130Unix November 4th, 2013 mbox(5)