maildir(5) Headers, Tables, and Macros maildir(5)
NAME
maildir - directory for incoming mail messages
INTRODUCTION
maildir is a structure for directories of incoming mail mes-
sages. It solves the reliability problems that plague mbox
files and mh folders.
RELIABILITY ISSUES
A machine may crash while it is delivering a message. For
both mbox files and mh folders this means that the message
will be silently truncated. Even worse: for mbox format, if
the message is truncated in the middle of a line, it will be
silently joined to the next message. The mail transport
agent will try again later to deliver the message, but it is
unacceptable that a corrupted message should show up at all.
In maildir, every message is guaranteed complete upon
delivery.
A machine may have two programs simultaneously delivering
mail to the same user. The mbox and mh formats require the
programs to update a single central file. If the programs
do not use some locking mechanism, the central file will be
corrupted. There are several mbox and mh locking mechan-
isms, none of which work portably and reliably. In con-
trast, in maildir, no locks are ever necessary. Different
delivery processes never touch the same file.
A user may try to delete messages from his mailbox at the
same moment that the machine delivers a new message. For
mbox and mh formats, the user's mail-reading program must
know what locking mechanism the mail-delivery programs use.
In contrast, in maildir, any delivered message can be safely
updated or deleted by a mail-reading program.
Many sites use Sun's Network Failure System (NFS), presum-
ably because the operating system vendor does not offer any-
thing else. NFS exacerbates all of the above problems.
Some NFS implementations don't provide any reliable locking
mechanism. With mbox and mh formats, if two machines
deliver mail to the same user, or if a user reads mail any-
where except the delivery machine, the user's mail is at
risk. maildir works without trouble over NFS.
THE MAILDIR STRUCTURE
A directory in maildir format has three subdirectories, all
on the same filesystem: tmp, new, and cur.
Each file in new is a newly delivered mail message. The
modification time of the file is the delivery date of the
message. The message is delivered without an extra UUCP-
style From_ line, without any >From quoting, and without an
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maildir(5) Headers, Tables, and Macros maildir(5)
extra blank line at the end. The message is normally in RFC
822 format, starting with a Return-Path line and a
Delivered-To line, but it could contain arbitrary binary
data. It might not even end with a newline.
Files in cur are just like files in new. The big difference
is that files in cur are no longer new mail: they have been
seen by the user's mail-reading program.
HOW A MESSAGE IS DELIVERED
The tmp directory is used to ensure reliable delivery, as
discussed here.
A program delivers a mail message in six steps. First, it
chdir()s to the maildir directory. Second, it stat()s the
name tmp/time.pid.host, where time is the number of seconds
since the beginning of 1970 GMT, pid is the program's pro-
cess ID, and host is the host name. Third, if stat()
returned anything other than ENOENT, the program sleeps for
two seconds, updates time, and tries the stat() again, a
limited number of times. Fourth, the program creates
tmp/time.pid.host. Fifth, the program NFS-writes the mes-
sage to the file. Sixth, the program link()s the file to
new/time.pid.host. At that instant the message has been
successfully delivered.
The delivery program is required to start a 24-hour timer
before creating tmp/time.pid.host, and to abort the delivery
if the timer expires. Upon error, timeout, or normal com-
pletion, the delivery program may attempt to unlink()
tmp/time.pid.host.
NFS-writing means (1) as usual, checking the number of bytes
returned from each write() call; (2) calling fsync() and
checking its return value; (3) calling close() and checking
its return value. (Standard NFS implementations handle
fsync() incorrectly but make up for it by abusing close().)
HOW A MESSAGE IS READ
A mail reader operates as follows.
It looks through the new directory for new messages. Say
there is a new message, new/unique. The reader may freely
display the contents of new/unique, delete new/unique, or
rename new/unique as cur/unique:info. See
http://pobox.com/~djb/maildir.html for the meaning of info.
The reader is also expected to look through the tmp direc-
tory and to clean up any old files found there. A file in
tmp may be safely removed if it has not been accessed in 36
hours.
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maildir(5) Headers, Tables, and Macros maildir(5)
It is a good idea for readers to skip all filenames in new
and cur starting with a dot. Other than this, readers
should not attempt to parse filenames.
ENVIRONMENT VARIABLES
Mail readers supporting maildir use the MAILDIR environment
variable as the name of the user's primary mail directory.
SEE ALSO
mbox(5), qmail-local(8)
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