man git-format-patch makes reference of the UNIX mailbox format which is a term I am unfamiliar with.

A google search for "UNIX mailbox format" and similar expressions lists many hits with the term mbox in it.

There is even a man page (man mbox) for mbox.

I am lead to conclude that mbox and the UNIX mailbox format are the same thing, but I am not 100% sure.

Can someone confirm (or deny) my assumption?

3 Answers 3


Can someone confirm (or deny) my assumption?

Yes, Both are same.

UNIX mbox format is used by AsyncOS when messages are archived (in anti-spam and anti-virus configuration) and logged (in the message filter log() action).

mbox is traditional UNIX mailbox format. Users' INBOX mboxes are commonly stored in /var/spool/mail or /var/mail directory. Single file contains multiple messages and is the most common format for storing email messages on a hard drive. All the messages for each mailbox are stored as a single, long, text file in a string of concatenated e-mail messages, starting with the “From” header of the message.

  • 3
    IMHO "the most common format" has been true. Multiple (small) messages in single file have made sense to preserve precious disk space. In era of bigger messages and cheaper disk space it is mostly "unwillingness to change" that keeps "one mail one file" (maildir) away.
    – AnFi
    May 2, 2016 at 11:20

The two are not equated in an RFC or other standard, but if you read further, you will notice that searching for the two terms together finds several articles which equate them, as well as pointing out a commonly used different format, maildir.

RFC 4155 (written in September 2005, which is recent) says

The mbox database format is not documented in an authoritative specification, but instead exists as a well-known output format that is anecdotally documented, or which is only authoritatively documented for a specific platform or tool.

Further reading:


In UNIX, the original mail and the Berkeley mail (in Sys-C UNIX the Berkeley MUA [mail user agent] was called mailx) stored messages in /var/spool/mail/username, and if you saved messages they were moved to your /home/username/MAIL file.

All the messages were in a single file separated by a blank line and starting with a single From - date_string line, called the mbox format. Note that each message also should also contain a From: line that is part of the standard RFC email format.

There were a couple of alternatives to mail, namely mh, elm and mutt. These used a directory hierarchy called maildir format, essentially using the filesystem as a database.

When Netscape Communicator came out, and later broke up into Firefox and Thunderbird, it retained the mbox format from UNIX and Linux, but maintained a different file for each "account" or source of email. Note that to retain the speed of using the filesystem style maildir format (yes it was faster for lots of emails), Netscape invented an index file ending in *.msf.

  • More information on the mbox format can be found at this LoC site
  • More information on the maildir format can be found e.g. here

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