1 .TH mbox 5

     2 .SH "NAME"

     3 mbox \- file containing mail messages

     4 .SH "INTRODUCTION"

     5 The most common format for storage of mail messages is

     6 .I mbox

     7 format.

     8 An

     9 .I mbox

    10 is a single file containing zero or more mail messages.

    11 .SH "MESSAGE FORMAT"

    12 A message encoded in

    13 .I mbox

    14 format begins with a

    15 .B From_

    16 line, continues with a series of

    17 .B \fRnon-\fBFrom_

    18 lines,

    19 and ends with a blank line.

    20 A

    21 .B From_

    22 line means any line that begins with the characters

    23 F, r, o, m, space:

    24 

    25 .EX

    26      From god@heaven.af.mil Sat Jan  3 01:05:34 1996

    27 .br

    28      Return-Path: <god@heaven.af.mil>

    29 .br

    30      Delivered-To: djb@silverton.berkeley.edu

    31 .br

    32      Date: 3 Jan 1996 01:05:34 -0000

    33 .br

    34      From: God <god@heaven.af.mil>

    35 .br

    36      To: djb@silverton.berkeley.edu (D. J. Bernstein)

    37 .br

    38 

    39 .br

    40      How's that mail system project coming along?

    41 .br

    42 

    43 .EE

    44 

    45 The final line is a completely blank line (no spaces or tabs).

    46 Notice that blank lines may also appear elsewhere in the message.

    47 

    48 The

    49 .B From_

    50 line always looks like

    51 .B From

    52 .I envsender

    53 .I date

    54 .IR moreinfo .

    55 .I envsender

    56 is one word, without spaces or tabs;

    57 it is usually the envelope sender of the message.

    58 .I date

    59 is the delivery date of the message.

    60 It always contains exactly 24 characters in

    61 .B asctime

    62 format.

    63 .I moreinfo

    64 is optional; it may contain arbitrary information.

    65 

    66 Between the

    67 .B From_

    68 line and the blank line is a message in RFC 822 format,

    69 as described in

    70 .BR qmail-header(5) ,

    71 subject to

    72 .B >From quoting

    73 as described below.

    74 .SH "HOW A MESSAGE IS DELIVERED"

    75 Here is how a program appends a message to an

    76 .I mbox

    77 file.

    78 

    79 It first creates a

    80 .B From_

    81 line given the message's envelope sender and the current date.

    82 If the envelope sender is empty (i.e., if this is a bounce message),

    83 the program uses

    84 .B MAILER-DAEMON

    85 instead.

    86 If the envelope sender contains spaces, tabs, or newlines,

    87 the program replaces them with hyphens.

    88 

    89 The program then copies the message, applying

    90 .B >From quoting

    91 to each line.

    92 .B >From quoting

    93 ensures that the resulting lines are not

    94 .B From_

    95 lines:

    96 the program prepends a

    97 .B >

    98 to any

    99 .B From_

   100 line,

   101 .B >From_

   102 line,

   103 .B >>From_

   104 line,

   105 .B >>>From_

   106 line,

   107 etc.

   108 

   109 Finally the program appends a blank line to the message.

   110 If the last line of the message was a partial line,

   111 it writes two newlines;

   112 otherwise it writes one.

   113 .SH "HOW A MESSAGE IS READ"

   114 A reader scans through an

   115 .I mbox

   116 file looking for

   117 .B From_

   118 lines.

   119 Any

   120 .B From_

   121 line marks the beginning of a message.

   122 The reader should not attempt to take advantage of the fact that every

   123 .B From_

   124 line (past the beginning of the file)

   125 is preceded by a blank line.

   126 

   127 Once the reader finds a message,

   128 it extracts a (possibly corrupted) envelope sender

   129 and delivery date out of the

   130 .B From_

   131 line.

   132 It then reads until the next

   133 .B From_

   134 line or end of file, whichever comes first.

   135 It strips off the final blank line

   136 and

   137 deletes the

   138 quoting of

   139 .B >From_

   140 lines and

   141 .B >>From_

   142 lines and so on.

   143 The result is an RFC 822 message.

   144 .SH "COMMON MBOX VARIANTS"

   145 There are many variants of

   146 .I mbox

   147 format.

   148 The variant described above is

   149 .I mboxrd

   150 format, popularized by Rahul Dhesi in June 1995.

   151 

   152 The original

   153 .I mboxo

   154 format quotes only

   155 .B From_

   156 lines, not

   157 .B >From_

   158 lines.

   159 As a result it is impossible to tell whether

   160 

   161 .EX

   162      From: djb@silverton.berkeley.edu (D. J. Bernstein)

   163 .br

   164      To: god@heaven.af.mil

   165 .br

   166 

   167 .br

   168      >From now through August I'll be doing beta testing.

   169 .br

   170      Thanks for your interest.

   171 .EE

   172 

   173 was quoted in the original message.

   174 An

   175 .I mboxrd

   176 reader will always strip off the quoting.

   177 

   178 .I mboxcl

   179 format is like

   180 .I mboxo

   181 format, but includes a Content-Length field with the 

   182 number of bytes in the message.

   183 .I mboxcl2

   184 format is like

   185 .I mboxcl

   186 but has no

   187 .B >From

   188 quoting.

   189 These formats are used by SVR4 mailers.

   190 .I mboxcl2

   191 cannot be read safely by

   192 .I mboxrd

   193 readers.

   194 .SH "UNSPECIFIED DETAILS"

   195 There are many locking mechanisms for

   196 .I mbox

   197 files.

   198 .B qmail-local

   199 always uses

   200 .B flock

   201 on systems that have it, otherwise

   202 .BR lockf .

   203 

   204 The delivery date in a

   205 .B From_

   206 line does not specify a time zone.

   207 .B qmail-local

   208 always creates the delivery date in GMT

   209 so that 

   210 .I mbox

   211 files can be safely transported from one time zone to another.

   212 

   213 If the mtime on a nonempty 

   214 .I mbox

   215 file is greater than the atime,

   216 the file has new mail.

   217 If the mtime is smaller than the atime,

   218 the new mail has been read.

   219 If the atime equals the mtime,

   220 there is no way to tell whether the file has new mail,

   221 since

   222 .B qmail-local

   223 takes much less than a second to run.

   224 One solution is for a mail reader to artificially set the

   225 atime to the mtime plus 1.

   226 Then the file has new mail if and only if the atime is

   227 less than or equal to the mtime.

   228 

   229 Some mail readers place

   230 .B Status

   231 fields in each message to indicate which messages have been read.

   232 .SH "SEE ALSO"

   233 maildir(5),

   234 qmail-header(5),

   235 qmail-local(8)