Maildir Mailbox Format

The Maildir format debuted with the qmail server in the mid-1990s. Each mailbox folder is a directory and each message a file. This improves efficiency because individual emails can be modified, deleted and added without affecting the mailbox or other emails, and makes it safer to use on networked file systems such as NFS.

For information on how to configure Maildir in Dovecot, see Maildir Configuration.

Dovecot Extensions

Since the Maildir standard doesn’t provide everything needed to fully support the IMAP protocol, Dovecot had to create some of its own non-standard extensions. The extensions still keep the Maildir standards compliant, so MUAs not supporting the extensions can still safely use it as a normal Maildir.

IMAP UID mapping

IMAP requires each message to have a permanent unique ID number. Dovecot uses the dovecot-uidlist file to keep UID <-> filename mapping. The file is basically in the same format as Courier IMAP’s courierimapuiddb file, except for one difference (see below).

The file begins with a header:

3 V1275660208 N25022 G3085f01b7f11094c501100008c4a11c1

  • 3 is the file format version number used by Dovecot v1.1+

  • 1275660208 is the IMAP UIDVALIDITY

  • 25022 is the UID that will be given to the next added message

  • 3085f01b7f11094c501100008c4a11c1 is the 128 bit mailbox global UID in hex

  • There may be other fields, and the order of these fields isn’t important

Version 1 file format is compatible with Courier. Version 2 was used by a few Dovecot non-release versions.

After the header comes the list of UID <-> filename mappings:

25006 :1276528487.M364837P9451.kurkku,S=1355,W=1394:2,
25017 W2481 :1276533073.M242911P3632.kurkku:2,F

  • 25006, 25017 are message UIDs

  • 2481 is the second message’s virtual size. First message contains it in the filename itself, so it’s not duplicated.

  • There may be more fields before ‘:’ character

  • Rest of the line after ‘:’ is the last known filename. This filename doesn’t necessarily exist currently, because the filename changes every time a message’s flags change. Dovecot doesn’t waste disk I/O by rewriting uidlist file every time flags change, but whenever it is rewritten the latest filenames are used. This allows Dovecot to try to guess what the message’s current filename is and if successful, avoid having to scan the directory’s contents.

The dovecot-uidlist file doesn’t need to be locked for reading. When writing, dovecot-uidlist.lock file needs to be created. New lines can be appended to the end of file, but existing data must never be directly modified; it can only be replaced with rename() system call.

dovecot-uidlist is updated lazily to optimize for disk I/O. If a message is expunged, it may not be removed from dovecot-uidlist until sometimes later. This means that if you create a new file using the same file name as what already exists in dovecot-uidlist, Dovecot thinks you “unexpunged” message by restoring a message from backup. This causes a warning to be logged and the file to be renamed.

Note that messages must not be modified once they’ve been delivered. IMAP (and Dovecot) requires that messages are immutable. If you wish to modify them in any way, create a new message instead and expunge the old one.

IMAP Keywords

All the non-standard message flags are called keywords in IMAP. Some clients use these automatically for marking spam (eg. $Junk, $!NonJunk, $Spam, $!NonSpam keywords). Thunderbird uses labels which map to keywords $Label1, $Label2, etc.

Dovecot stores keywords in the Maildir filename’s flags field using letters a..z. This means that only 26 keywords are possible to store in the Maildir. If more are used, they’re still stored in Dovecot’s index files. The mapping from single letters to keyword names is stored in dovecot-keywords file. The file is in format:

0 $Junk
1 $NonJunk

0 means letter a in the Maildir filename, 1 means b, and so on. The file doesn’t need to be locked for reading, but when writing dovecot-uidlist file must be locked. The file must not be directly modified; it can only be replaced with rename() system call.

For example, a file named

1234567890.M20046P2137.mailserver,S=4542,W=4642:2,Sb

would be flagged as $NonJunk with the above keywords.

Maildir Filename Extensions

The standard filename definition is: <base filename>:2,<flags>. Dovecot has extended the <flags> field to be <flags>[,<non-standard fields>]. This means that if Dovecot sees a comma in the <flags> field while updating flags in the filename, it doesn’t touch anything after the comma. However other Maildir MUAs may mess them up, so it’s still not such a good idea to do that. Basic <flags> are described in the Maildir standard. The <non-standard fields> isn’t used by Dovecot for anything currently.

Dovecot supports reading a few fields from the <base filename>:

  • ,S=<size>: <size> contains the file size. Getting the size from the filename avoids doing a system stat() call, which may improve the performance. This is especially useful with Quota Backend: maildir.

  • ,W=<vsize>: <vsize> contains the file’s RFC822.SIZE, i.e., the file size with linefeeds being CR+LF characters. If the message was stored with CR+LF linefeeds, <size> and <vsize> are the same. Setting this may give a small speedup because now Dovecot doesn’t need to calculate the size itself.

A Maildir filename with those fields would look something like:

1035478339.27041_118.foo.org,S=1000,W=1030:2,S

Usage of Timestamps

Timestamps of message files:

  • mtime is used as IMAP INTERNALDATE RFC 3501#section-2.3.3, and must never change (see RFC 3501 [2.3.1.1, parenthesis 4]).

  • ctime is used as Dovecot’s internal “save/copy date”, unless the correct value is found from dovecot.index.cache. This is used only by external commands, e.g. doveadm expunge savedbefore.

  • atime is not used.

Timestamps of cur and new directories:

  • mtime is used to detect changes of the mailbox and may force regeneration of index files.

  • atime and ctime not used.

Filename Examples

Filename

Explanation

1491941793.M41850P8566V0000000000000015I0000000004F3030E_0.mx1.example.com,S=10956:2,STln

UNIX timestamp of arrival

1491941793.M41850P8566V0000000000000015I0000000004F3030E_0.mx1.example.com,S=10956:2,STln

Size of e-mail

1491941793.M41850P8566V0000000000000015I0000000004F3030E_0.mx1.example.com,S=10956:2,STln

S = seen (marked as read)

T = trashed

l = IMAP tag #12 (0=a, 1=b, 2=c, etc) as defined in that folder’s dovecot-keywords file.

n = IMAP tag #14 (0=a, 1=b, 2=c, etc) as defined in that folder’s dovecot-keywords file.

Maildir and Filesystems

Note

Information in this section is old/dated. It remains here for informational purposes, but it is recommended that newer filesystems (e.g. ext4, JFS, ZFS, btrfs, etc.) be evaluated as they may contain technical improvements that workaround the limitations discuss below.

General Comparisons of Maildir on Different Filesystems

Linux ext2 / ext3

The main disadvantage in using these filesystems is that searching can be slightly slower, and access to very large mailboxes (thousands of messages) can get slow with filesystems which don’t have directory indexes.

Old versions of ext2 and ext3 on Linux don’t support directory indexing (to speed up access), but newer versions of ext3 do, although you may have to manually enable it. You can check if the indexing is already enabled with tune2fs:

tune2fs -l /dev/hda3 | grep features

If you see dir_index, you’re all set. If dir_index is missing, add it using:

umount /dev/hda3
tune2fs -O dir_index /dev/hda3
e2fsck -fD /dev/hda3
mount /dev/hda3

XFS

XFS performance seems to depend on a lot of factors, also on the system and the file system parameters.

Various Tips

NFS

NFS v3 performance can be adversely affected by readdirplus, which causes the NFS server to stat() every file in a directory. The solution under Linux is to make sure the NFS filesystem is mounted with the nordirplus option.

See: https://dovecot.org/list/dovecot/2012-July/066939.html

Directory Structure

By default Dovecot uses the Maildir++ directory layout for organizing mailbox directories. This means that all the folders are directly inside ~/Maildir directory:

  • ~/Maildir/new, ~/Maildir/cur and ~/Maildir/tmp directories contain the messages for INBOX. The tmp directory is used during delivery, new messages arrive in new, and read messages are moved to cur by the clients.

  • ~/Maildir/.folder/ is a mailbox folder.

  • ~/Maildir/.folder.subfolder/ is a subfolder of a folder (i.e. folder/subfolder).

You can also optionally use the fs layout by appending :LAYOUT=fs to mail_location. This makes the folder structure look like:

  • ~/Maildir/new, ~/Maildir/cur and ~/Maildir/tmp directories contain the messages for INBOX, just like with Maildir++.

  • ~/Maildir/folder/ is a mailbox folder.

  • ~/Maildir/folder/subfolder/ is a subfolder of a folder.

Filesystem Permissions

See Shared Mailboxes Permissions for how permissions are set for newly created files and directories.

Issues with the Specification

Locking

Although Maildir was designed to be lockless, Dovecot locks the Maildir while doing modifications to it or while looking for new messages in it. This is required because otherwise Dovecot might temporarily see mails incorrectly deleted, which would cause trouble. Basically the problem is that if one process modifies the Maildir (eg. a rename() to change a message’s flag), another process in the middle of listing files at the same time could skip a file. The skipping happens because readdir() system call doesn’t guarantee that all the files are returned if the directory is modified between the calls to it. This problem exists with all the commonly used filesystems.

Because Dovecot uses its own non-standard locking (dovecot-uidlist.lock dotlock file), other MUAs accessing the Maildir don’t support it. This means that if another MUA is updating message flags or expunging messages, Dovecot might temporarily lose some message(s). After the next sync when it finds it again, an error message may be written to log and the message will receive a new UID.

Delivering mails to new/ directory doesn’t have any problems, so there’s no need for LDAs to support any type of locking.

Mail Delivery

Qmail’s how a message is delivered page suggests to deliver the mail like this:

  1. Create a unique filename (only time.pid.host here, later Maildir spec has been updated to allow more uniqueness identifiers)

  2. Do stat(tmp/<filename>). If the stat() found a file, wait 2 seconds and go back to step 1.

  3. Create and write the message to tmp/<filename>.

  4. link() it into new/ directory. Although not mentioned here, the link() could again fail if the mail existed in new/ dir. In that case you should probably go back to step 1.

All this trouble is rather pointless. Only the first step is what really guarantees that the mails won’t get overwritten, the rest just sounds nice. Even though they might catch a problem once in a while, they give no guaranteed protection and will just as easily pass duplicate filenames through and overwrite existing mails.

Step 2 is pointless because there’s a race condition between steps 2 and 3. PID/host combination by itself should already guarantee that it never finds such a file. If it does, something’s broken and the stat() check won’t help since another process might be doing the same thing at the same time, and you end up writing to the same file in tmp/, causing the mail to get corrupted.

In step 4 the link() would fail if an identical file already existed in the Maildir, right? Wrong. The file may already have been moved to cur/ directory, and since it may contain any number of flags by then you can’t check with a simple stat() anymore if it exists or not.

Step 2 was pointed out to be useful if clock had moved backwards. However, this doesn’t give any actual safety guarantees because an identical base filename could already exist in cur/. Besides if the system was just rebooted, the file in tmp/ could probably be even overwritten safely (assuming it wasn’t already link()ed to new/).

So really, all that’s important in not getting mails overwritten in your Maildir is step 1: Always create filenames that are guaranteed to be unique. Forget about the 2 second waits and such that the Qmail’s man page talks about.

Maildir and Mail Header Metadata

Unlike when using mbox as mailbox format, where mail headers (for example Status, X-UID, etc.) are used to determine and store metadata, the mail headers within Maildir files are (usually) not used for this purpose by Dovecot; neither when mails are created/moved/etc. via IMAP nor when Maildirs are placed (e.g., copied or moved in the filesystem) in a mail location (and then “imported” by dovecot).

Therefore, it is (usually) not necessary, to strip any such mail headers at the MTA, MDA, or LDA (as is recommended with mbox).

There is one exception, though, namely when pop3_reuse_xuidl = yes (which is however deprecated): in this case X-UIDL is used for the POP3 UIDLs. Therefore, in this case, is recommended to strip the X-UIDL mail headers case-insensitively at the MTA, MDA, or LDA.

Procmail Problems

Maildir format is somewhat compatible with MH format. This is sometimes a problem when people configure their procmail to deliver mails to Maildir/new. This makes procmail create the messages in MH format, which basically means that the file is called msg.inode_number. While this appears to work first, after expunging messages from the Maildir the inodes are freed and will be reused later. This means that another file with the same name may come to the Maildir, which makes Dovecot think that an expunged file reappeared into the mailbox and an error is logged.

The proper way to configure procmail to deliver to a Maildir is to use Maildir/ as the destination.

References