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dovecot.md

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Mailbox, IMAP, Dovecot

Target audience

People who find themselves with a maildir (dump) and try to make heads or tail of it

Motivation

The internet has fragments of interest but nothing really clear, and the dovecot docs are currently down

Maildir

The core description is simple enough: at the core, a maildir is a folder representing a mailbox using three sub-folders, new, tmp, and cur:

  • tmp is the mail server's scratch pad, and more generally equivalent to /tmp
  • new is for messages which haven't yet been given to mail clients messages generally go into new after having been created in tmp
  • cur is for messages which have been fetched at least once

Unless the mailbox is very high traffic, odds are good tmp and new will be empty.

Each subdir then contains a bunch of files, one message per files. Most of the name is essentially random and irrelevant so we'll ignore it, the only interesting bit is the trailer, which is the info segment: a colon (:), the character 2, a comma (,), and a bunch of letters. Each letter is a flag.

The original specs defines 6 flags:

  • P(assed): the message has been resent/bounced/forwarded
  • R(eplied): the message has been replied to
  • S(een): the message has been seen
  • T(rashed): the message has been moved to the trash
  • D(raft): the message is a draft
  • F(lagged): no intrinsic meaning

A maildir can have folders, these are directories prefixed by . and siblings of cur. They can be nested but their nesting is logical rather than physical e.g. .foo.bar is the folder foo/bar.

IMAP

There are pages and pages of IMAP spec, the only bits of interest currently are the notions of flags and keywords.

IMAP has system flags, these are prefixed by \ and defined by RFC 2060:

  • \Seen
  • \Answered
  • \Flagged
  • \Deleted
  • \Draft
  • \Recent

5 of those map fine to Maildir flags (Maildir also has P while IMAP has \Recent).

Keywords are anything attribute which does not start with a \. They are server-specified, but a server may allow clients to define new keywords by returning \* as one of the PERMANENTFLAGS.

Following RFC 5788 (but also inheriting the choices from RFC 3503), all $-prefixed keywords are also reserved for official registration. Only non-prefixed keywords can have arbirary semantics.

Note an odd conflict: common use keywords can get registered officially, leading to duplicate behaviour. That is the case of Junk/NotJunk and $Junk/$NotJunk, the latter replaces the former after registration, as a result it can be useful to support both.

Also INBOX is the primary mailbox (the maildir root), maildir folders would correspond to IMAP mailboxes.

dovecot

dovecot is an IMAP server which uses maildir but extends it a lot in order to support IMAP semantics (for which the base maildir is insufficient). A dovecot maildir is recognizable because the root directory has a lot of dovecot-prefixed files at the root (next to the cur, tmp, and new directories).

The extensions are documented but right now the entire dovecot docs / wiki is down so I figure I'll write down what I understand.

The most relevant bits for reading a dump are:

dovecot-keywords

As a rule, dovecot maildir files have more flags than standard. dovecot uses lowercase letters to denote its extensions, but because clients can define their own keywords the mapping is dynamic and mailbox-specific.

dovecot-keywords is an ASCII file containing a list (one item per line) of a decimal number (0-25) and an IMAP keyword. The number is the index of a letter (a-z) used to specify the corresponding keyword on the file.

So for instance given

0 $NotJunk
1 $Junk
2 NotJunk
3 JunkRecorded
4 Junk

a file could be marked ,Sac, meaning it has been Seen, and is flagged as $NotJunk and NotJunk. But given

0 $NotJunk
1 NotJunk
2 $Junk
3 JunkRecorded
4 Junk

the exact same file would be ,Sab instead.

dovecot-uidlist

Each IMAP message is supposed to have a unique id, in a format different than maildir filenames. This is where dovecot maintains the mapping. uidlist has a line of followed by lines of:

  • the message id
  • W and the "virtual size" (RFC822.SIZE)
  • possibly other fields
  • : and the filename, probably excluding the flags