A scheme used to store email messages on disk. Most MailBoxFormat?s use some variation on the following schemes

• FlatFile - mbox? is the most common example of this, where all the messages in a mailbox are encoded into a single file.
• MessagesAsFiles? - MH, and MailDir are common examples of this scheme, where the mailbox is a directory, and each message is encoded as a single file.
• DataBase - MicrosoftExchange does this, as well as DBMail on Unix

There are, of course hybrid approaches, and, of course most of these approaches have various workarounds (indexing, offset tables) to overcome the performance problems that they suffer from. Alas, all of these workarounds and differing approaches tend to be application specific, which makes the vanilla formats more practical most of the time.

For a comparasion of several different schemes, have a read of http://www.washington.edu/imap/documentation/formats.txt.html, or for an even more subjective "discussion", http://slashdot.org/article.pl?sid=01/01/27/0138202 ;)