1MAILDIR(5) Double Precision, Inc. MAILDIR(5)
2
6 maildir - E-mail directory
7
9 $HOME/Maildir
10
12 A “Maildir” is a structured directory that holds E-mail messages.
13 Maildirs were first implemented by the Qmail mail server. Qmail's
14 maildirs were a simple data structure, nothing more than a single
15 collection of E-mail messages. The Courier mail server builds upon
16 Qmail's maildirs to provide extended functionality, such as folders and
17 quotas. This document describes the Courier mail server's extended
18 maildirs, without explicitly identifying The Courier mail
19 server-specific extensions. See maildir(5) in Qmail's documentation for
20 the original definition of maildirs.
21 Traditionally, E-mail folders were saved as plain text files, called
23 “mboxes”. Mboxes have known limitations. Only one application can use
24 an mbox at the same time. Locking is required in order to allow
25 simultaneous concurrent access by different applications. Locking is
26 often problematic, and not very reliable in network-based filesystem
27 requirements. Some network-based filesystems don't offer any reliable
28 locking mechanism at all. Furthermore, even bulletproof locking won't
29 prevent occasional mbox corruption. A process can be killed or
30 terminated in the middle of updating an mbox. This will likely result
31 in corruption, and a loss of most messages in the mbox.
32 Maildirs allow multiple concurrent access by different applications.
34 Maildirs do not require locking. Multiple applications can update a
35 maildir at the same time, without stepping on each other's feet.
36 Maildir contents
38 A “maildir” is a directory that's created by maildirmake(1)[1].
39 Naturally, maildirs should not have any group or world permissions,
40 unless you want other people to read your mail. A maildir contains
41 three subdirectories: tmp, new, and cur. These three subdirectories
42 comprise the primary folder, where new mail is delivered by the system.
43 Folders are additional subdirectories in the maildir whose names begin
45 with a period: such as .Drafts or .Sent. Each folder itself contains
46 the same three subdirectories, tmp, new, and cur, and an additional
47 zero-length file named maildirfolder, whose purpose is to inform any
48 mail delivery agent that it's really delivering to a folder, and that
49 the mail delivery agent should look in the parent directory for any
50 maildir-related information.
51 Folders are not physically nested. A folder subdirectory, such as .Sent
53 does not itself contain any subfolders. The main maildir contains a
54 single, flat list of subfolders. These folders are logically nested,
55 and periods serve to separate folder hierarchies. For example,
56 .Sent.2002 is considered to be a subfolder called “2002” which is a
57 subfolder of “Sent”.
58 Folder name encoding
60 Folder names can contain any Unicode character, except for control
61 characters. US-ASCII characters, U+0x0020 - U+0x007F, except for
62 the period, and forward-slash. Non-Latin characters are encoded in
63 UTF-8.
64 Other maildir contents
66 Software that uses maildirs may also create additional files
67 besides the tmp, new, and cur subdirectories -- in the main maildir
68 or a subfolder -- for its own purposes.
69 Messages
71 E-mail messages are stored in separate, individual files, one E-mail
72 message per file. The tmp subdirectory temporarily stores E-mail
73 messages that are in the process of being delivered to this maildir.
74 tmp may also store other kinds of temporary files, as long as they are
75 created in the same way that message files are created in tmp. The new
76 subdirectory stores messages that have been delivered to this maildir,
77 but have not yet been seen by any mail application. The cur
78 subdirectory stores messages that have already been seen by mail
79 applications.
80 Adding new mail to maildirs
82 The following process delivers a new message to the maildir:
83 A new unique filename is created using one of two possible forms:
85 “time.MusecPpid.host”, or “time.MusecPpid_unique.host”. “time” and
86 “usec” is the current system time, obtained from gettimeofday(2).
87 “pid” is the process number of the process that is delivering this
88 message to the maildir. “host” is the name of the machine where the
89 mail is being delivered. In the event that the same process creates
90 multiple messages, a suffix unique to each message is appended to the
91 process id; preferrably an underscore, followed by an increasing
92 counter. This applies whether messages created by a process are all
93 added to the same, or different, maildirs. This protocol allows
94 multiple processes running on multiple machines on the same network to
95 simultaneously create new messages without stomping on each other.
96 The filename created in the previous step is checked for existence by
98 executing the stat(2) system call. If stat(2) results in ANYTHING OTHER
99 than the system error ENOENT, the process must sleep for two seconds,
100 then go back and create another unique filename. This is an extra step
101 to insure that each new message has a completely unique filename.
102 Other applications that wish to use tmp for temporary storage should
104 observe the same protocol (but see READING MAIL FROM MAILDIRS below,
105 because old files in tmp will be eventually deleted).
106 If the stat(2) system call returned ENOENT, the process may proceed to
108 create the file in the tmp subdirectory, and save the entire message in
109 the new file. The message saved MUST NOT have the “From_” header that
110 is used to mboxes. The message also MUST NOT have any “From_” lines in
111 the contents of the message prefixed by the “>” character.
112 When saving the message, the number of bytes returned by the write(2)
114 system call must be checked, in order to make sure that the complete
115 message has been written out.
116 After the message is saved, the file descriptor is fstat(2)-ed. The
118 file's device number, inode number, and the its byte size, are saved.
119 The file is closed and is then immediately moved/renamed into the new
120 subdirectory. The name of the file in new should be
121 “time.MusecPpidVdevIino.host,S=cnt”, or
122 “time.MusecPpidVdevIino_unique.host,S=cnt”. “dev” is the message's
123 device number, “ino” is the message's inode number (from the previous
124 fstat(2) call); and “cnt” is the message's size, in bytes.
125 The “,S=cnt” part optimizes the Courier[2] mail server's maildir quota
127 enhancement; it allows the size of all the mail stored in the maildir
128 to be added up without issuing the stat(2) system call for each
129 individual message (this can be quite a performance drain with certain
130 network filesystems).
131 READING MAIL FROM MAILDIRS
133 Applications that read mail from maildirs should do it in the following
134 order:
135 When opening a maildir or a maildir folder, read the tmp subdirectory
137 and delete any files in there that are at least 36 hours old.
138 Look for new messages in the new subdirectory. Rename new/filename, as
140 cur/filename:2,info. Here, info represents the state of the message,
141 and it consists of zero or more boolean flags chosen from the
142 following: “D” - this is a 'draft' message, “R” - this message has been
143 replied to, “S” - this message has been viewed (seen), “T” - this
144 message has been marked to be deleted (trashed), but is not yet removed
145 (messages are removed from maildirs simply by deleting their file), “F”
146 - this message has been marked by the user, for some purpose. These
147 flags must be stored in alphabetical order. New messages contain only
148 the :2, suffix, with no flags, indicating that the messages were not
149 seen, replied, marked, or deleted.
150 Maildirs may have maximum size quotas defined, but these quotas are
152 purely voluntary. If you need to implement mandatory quotas, you should
153 use any quota facilities provided by the underlying filesystem that is
154 used to store the maildirs. The maildir quota enhancement is designed
155 to be used in certain situations where filesystem-based quotas cannot
156 be used for some reason. The implementation is designed to avoid the
157 use of any locking. As such, at certain times the calculated quota may
158 be imprecise, and certain anomalous situations may result in the
159 maildir actually going over the stated quota. One such situation would
160 be when applications create messages without updating the quota
161 estimate for the maildir. Eventually it will be precisely recalculated,
162 but wherever possible new messages should be created in compliance with
163 the voluntary quota protocol.
164 The voluntary quota protocol involves some additional procedures that
166 must be followed when creating or deleting messages within a given
167 maildir or its subfolders. The deliverquota(8)[3] command is a tiny
168 application that delivers a single message to a maildir using the
169 voluntary quota protocol, and hopefully it can be used as a measure of
170 last resort. Alternatively, applications can use the libmaildir.a
171 library to handle all the low-level dirty details for them. The
172 voluntary quota enhancement is described in the maildirquota(7)[4] man
173 page.
174 Maildir Quotas
176 This is a voluntary mechanism for enforcing "loose" quotas on the
177 maximum sizes of maildirs. This mechanism is enforced in software, and
178 not by the operating system. Therefore it is only effective as long as
179 the maildirs themselves are not directly accessible by their users,
180 since this mechanism is trivially disabled.
181 If possible, operating system-enforced quotas are preferrable. Where
183 operating system quota enforcement is not available, or not possible,
184 this voluntary quota enforcement mechanism might be an acceptable
185 compromise. Since it's enforced in software, all software that modifies
186 or accesses the maildirs is required to voluntary obey and enforce a
187 quota. The voluntary quota implementation is flexible enough to allow
188 non quota-aware applications to also access the maildirs, without any
189 drastic consequences. There will be some non-drastic consequences,
190 though. Of course, non quota-aware applications will not enforce any
191 defined quotas. Furthermore, this voluntary maildir quota mechanism
192 works by estimating the current size of the maildir, with periodic
193 exact recalculation. Obviously non quota-aware maildir applications
194 will not update the maildir size estimation, so the estimate will be
195 thrown off for some period of time, until the next recalculation.
196 This voluntary quota mechanism is designed to be a reasonable
198 compromise between effectiveness, and performance. The entire purpose
199 of using maildir-based mail storage is to avoid any kind of locking,
200 and to permit parallel access to mail by multiple applications. In
201 order to compute the exact size of a maildir, the maildir must be
202 locked somehow to prevent any modifications while its contents are
203 added up. Obviously something like that defeats the original purpose of
204 using maildirs, therefore the voluntary quota mechanism does not use
205 locking, and that's why the current recorded maildir size is always
206 considered to be an estimate. Regular size recalculations will
207 compensate for any occasional race conditions that result in the
208 estimate to be thrown off.
209 A quota for an existing maildir is installed by running maildirmake
211 with the -q option, and naming an existing maildir. The -q option takes
212 a parameter, quota, which is a comma-separated list of quota
213 specifications. A quota specification consists of a number followed by
214 either 'S', indicating the maximum message size in bytes, or 'C',
215 maximum number of messages. For example:
216 maildirmake -q 5000000S,1000C ./Maildir
218 This sets the quota to 5,000,000 bytes or 1000 messages, whichever
220 comes first.
221 maildirmake -q 1000000S ./Maildir
223 This sets the quota to 1,000,000 bytes, without limiting the number of
225 messages.
226 A quota of an existing maildir can be changed by rerunning the
228 maildirmake command with a new -q option. To delete a quota entirely,
229 delete the Maildir/maildirsize file.
230
232 maildirmake(1)[1].
233
235 Sam Varshavchik
236 Author
237
239 1. maildirmake(1)
240 http://www.courier-mta.org/maildirmake.html
241 2. Courier
243 http://www.courier-mta.org
244 3. deliverquota(8)
246 http://www.courier-mta.org/deliverquota.html
247 4. maildirquota(7)
249 http://www.courier-mta.org/maildirquota.html
250Courier Mail Server 09/24/2019 MAILDIR(5)