Mailboxes

Overview

To (over)use the analogy once again of a postal system, we think of a mailbox as a place where a letter is placed to await pickup by its recipient. Although current methods have evolved considerably, the original idea used for the development of email was exactly that. The concept has been around officially since at least 1971. The image of a mailbox is so prevalent that, although current systems have evolved to do so much more than just hold messages for processing, the industry continues to apply the concept.

The more general term is "Email Storage". It is necessary to store email for many reasons:

  • As a mailbox system — part of a queue to await processing

  • As a persistance mechanism — a place where you can go back and predictably locate a message that you may want to refer to again in the future

  • As an archive — a place where you may want to store "deleted" emails so that you can potentially retrieve them if you wish to change your mind later

  • As a temporary step in a process — perhaps you have a "todo" folder where you temporarily store an email that you intend to process soon

There are numerous ways to store emails, just as there are numerous reasons for storing them:

  • A standardized "mailbox" format

  • A relational database

  • A custom file-based mechanism

  • Etc.

This section briefly describes how email storage is a core concept in an email system.

Specifications

Electronic Mail Box

RFC196

The concept of an "electronic mail box" was initially specified way back in 1971. It is amusing to read the retro concepts in the initial description of this document:

The purpose of this protocol is to provide at each site a
standard mechanism to receive sequential files for immediate or
deferred printing or other uses.  The files for deferred printing
would probably be stored on intermediate disk files [...].
It is also assumed that there would be a program at the sending
site [...]. This program could probably be accessed as a subcommand
of the Telnet program.

This specification was obsoleted by RFC221.

RFC221

RFC221 ("A Mail Box Protocol, Version-2") was published in 1971. It added the possibility of using FTP in addition to the "Data Transfer Protocol" used in RFC196. This version was obsoleted by RFC278.

RFC278

RFC278 ("Revision of the Mail Box Protocol") was published in 1971. It provided a number of updates to RFC221.

Maildir

Maildir is a file-based storage format invented by Dan Bernstein. A major design objective was apparently to delegate file locking to the operating system.

Mbox

The mbox email storage format was formally defined by RFC4155. It is a formalization of a de facto format used by UNIX-like operating systems.

MIX

The MIX email storage format was developed by Mark Crispin, the original author of the IMAP specification. Its design goals were:

  • greater robustness against corruption caused by hardware or software failures. Many failures are "self-healing".

  • far fewer risky random-access I/O operations; a single false pointer calculation in other formats will corrupt the mailbox.

  • greater ease to repair damaged mailboxes.

  • (much) greater performance.

  • extensibility for new IMAP capabilities such as annotations, conditional store, or more aggressing caching.

RFC5322

RFC5322 weighs in on how to describe a "mailbox":

A mailbox receives mail. It is a 'conceptual entity' that does not necessarily
pertain to file storage. It further exemplifies that some sites may choose to
print mail on a printer and deliver the output to the addressee's desk, much
like a traditional fax transmission.

Indexing

Once a system grows over time it will likely contain many messages stored in its Mailboxes. Searching for the right email becomes increasingly difficult. At some point it becomes useful to use an indexing

Related Concepts

Email Storage is related to…​

  • POP, as the POP protocol mandates interaction with a user’s "mailbox"

  • IMAP, as the IMAP protocol is all about storing messages on an IMAP Server

  • Email clients, as the client will store mail locally, usually in the form of a "mailbox"

  • SMTP as the protocol is related to transmitting messages from one mailbox to another