Tuesday 5 March 2013
3/05/2013 01:15:00 am 1

What is .MBOX file format?

Email is the most efficient source of communication among business organizations for inside and outside communication both. As the business needs are increasing, various kind of Email clients are coming into existence .Each email client uses some specific file format for storing their mail database.

You might have heard of the file format .MBOX. This file format is used by many of the Email clients (such as Thunderbird, Entourage & Eudora etc.) for storing and organizing mails. 

MBOX format:
MBOX is a plain text file which contains the email as 7-bit ASCII text and the rest things such as attachments are stored in encoded format.

In MBOX format all mail messages are concatenated and stored in a single plain text file.
The beginning of each message is indicated by "From_" i.e. “From” followed by a white space character. The finish of each message is indicated by with a blank line appended at the end.

MBOX family:
The MBOX family contains several modified formats.
The four popular variants of MBOX format are:
mboxo, mboxrd, mboxcl, and mboxcl2.
  • mboxo:It is a variation of the original Mbox format in which the constant string "???@???" is used in place of sender's e-mail address.
Eudora is one of the popular email clients using mboxo format. In this case the messages are not saved intact as they were received as the attachments in the messages are separated and kept in a folder separately as individual files.
  • Mboxrd: It is a variation of mboxo mailbox format. The starting of a message is indicated by a "From” string at the beginning of a line in either the header or the body of a message.
The popular among the mboxrd using email clients are “qmail” and other Mozilla family of MUAs such as Netscape, Mozilla & Thunderbird, etc
  • mboxcl: mboxcl and mboxcl2 mailbox formats were used by Unix 'System V' mailboxes.
When a new mail needs to be appended to the "mboxcl" mailbox file, it is transformed before appending to the file. The mail message has a single '>' character pre appended to any line of the message, in either the header or the body, that begins with the five characters 'F', 'r', 'o', 'm'.

For locating the start of the message From_ line scanning is not used, instead of it the mboxcl uses a Content-Length: header. It denotes the length of the message body, after transformation, in octets. That helps in locating the start and end of a message.
  • mboxcl2:"mboxcl2" mailbox format is similar to mboxcl. It does not use any “From quoting", it also uses Content-Length: header for denoting he start and end of a message. In case of mboxcl2 no transformation is done before a message is being appended to an mboxcl2 mailbox file.
Default Location of MBOX File Format:
The MBOX files can be found at the default location:
“/User/Library/Mail/Mailboxes/ directory”
However, this location may differ from client to client.

MBOX supporting clients: The common MBOX using clients are: Thunderbird, Entourage, AppleMail, Eudora, Mulberry, SeaMonkey, Spicebird, Pocomail, Netscape, Mozillamail, Evolution, Sylpheed, Cone, Mutt, Gnu Mail, ClawsMail, OperaMail etc. To know more MBOX supporting clients visit:

While working with a particular Email client, many situations may arrive when you need to use the Email files from one email client into another email client. First thing you will do is trying to import the mailbox file from one Email client to another Email client with the help of its Import wizard. But all mailbox file formats are not inter-operable and compatible with all Email clients.

Consider a situation when you need to use the MBOX file into Outlook. How will you accomplish the migration task? Quite obvious, you will search for some method to import the MBOX file into Outlook. But Outlook doesn’t support MBOX mailbox format. So, you need to convert the MBOX file to PST format that is supported by Outlook. For converting the MBOX to PST format, you require some MBOX to PST converter software can find online.

PS: Any online software purchase to convert MBOX files to PST is the matter of choice, we advise you to read the company performance and software specification before buying any software.


  1. Quite a good piece of information on migration of MBOX files to MS Outlook. Thanks for the extra information on MBOX format and MBOX family.