mbox is an email storage format where all message is kept in single file. New message was appended below the last message.
Mbox is traditional UNIX mailbox format with single file contains zero or more messages. The relevant RFC about mbox format was RFC 4155. The other resource of this mbox format can be found at qmail man pages.
A message encoded in mbox format begins with a From line, continues with a series of non-From lines, and ends with a blank line. A From line means any line that begins with the characters F, r, o, m, space. Here the example of two message in one mbox file.
From someone@example.com Sat Jan 3 01:05:34 1996
Return-Path: <someone@example.com>
Delivered-To: somepeople@example.net
Date: 3 Jan 1996 01:05:34 -0000
From: S <someone@example.com>
To: somepeople@example.net (SP)
Hello world
From someone@example.com Sat Jan 3 01:05:34 1996
Return-Path: <root@example.org>
Delivered-To: somepeople@example.net
Date: 3 Jan 1996 03:01:49 -0000
From: S <root@example.org>
To: somepeople@example.net (SP)
The report has been delivered
When one process is reading or writing an email, it must lock the mbox file and prevents the other to modifying the mbox file. Because of this, mbox format does not work well with network mounted file systems, such as the Network File System (NFS).
