What is MBOX File? & Solution to Fix "Mailbox isn't a valid MBOX File" Error

Technical Description of .mbox File Extension

Developer Mozilla
File Signature 00 0D BB A0
Location Not defined
MIME Type Text/x-mailbox
File Category Email file
Latency Monitoring Zero-Latency Monitoring
Format Type Text and Binary
Analogy Type Analog Input & Analog Output
Supporting OS Windows, Linux, Unix, and Mac

Full Form of MBOX File Format

Mail Box File

MailBox Format: Description, Structure, & Location of .mbox File

What is MBOX format? - It is an email file type, which stores messages in plain text format. The file comprises of email content in form of 7-bit ASCII text and rest of email components (like metadata, attachments, etc.) are stored in encoded form. The MBOX file is supported or created by several email applications like SeaMonkey, Thunderbird, Entourage, Eudora, Apple Mail, Horde, etc. All emails of an email client are packed together and collectively saved in .mbox file extension.

mbox file extension

MBOX File Structure : In a string of concatenated email messages, all mail contents of mailboxes are stored like a long plain text file. The initial point of a message is indicated as From followed by white space character and the ending part of the same is denoted by a blank link. Well, there are some variants of MBOX file, which are described below:

Mboxo: It is somewhat different from original MailBox file because in section of senders mail address, it uses a constant string like “???@???”.

Mboxrd: This variant is an extended form of Mboxo file, where the initial point of message is described either in form of header or body.

Mboxcl: This type of MBOX file is operated in Unix ‘System V’ mailboxes

Mboxcl2: The last variant of MBOX is an enhanced version of Mboxcl file. For determining the email separator, it does not scan for ‘From’ lines. Each Mboxcl2 comprises of Content-Length attribute in its header part.

MBOX File Location : A MBOX is an email file format, which is generated from different email applications on various platforms. Therefore, there is no native location where one can find MBOX file. Well, depending upon the email application and platform, users can get MBOX file on their Machine.

MBOX File Location on Mac : Go through following set of instructions to determine MBOX file on Mac OS X Mail Edition 1:

  • 1. Launch Finder on your Apple Machine and then click on Go >> Go to folder… from menu list
  • 2. Either type ~/Library/Mail/V2 for Mac OS X Mail 5-8 or ~/Library/Mail/V3 for Mail, respectively.

Location of MBOX File In Windows : Follow below-mentioned steps to determine Thunderbird data file location:

  • 1. Open Run window on your machine and then execute %APPDATA%
  • 2. From the current menu list, click on Thunderbird folder and then on Profiles folder
  • 3. Each folder which is appearing within profiles folder is a profile of your system

Major Issues Faced With MBOX File

While working with MBOX file, email client users might encounter certain errors, which could delay entire procedure. Some of them are highlighted below:

  • Unable to open emails
  • Repairing data items of .mbox
  • Oversized MBOX file
  • Unable to load emails
  • Mailbox isn’t a valid MBOX file

Among all the above-mentioned error, here we are going to discuss solutions to fix mailbox isn’t a valid MBOX file error.

Recommendation: If it is necessary to access MBOX data into other email clients like Outlook, then you have to export data from MBOX to PST. To perform this task you need our trusted, user-friendly

Solution to Fix "Mailbox isn't a valid MBOX file" Error

The reason behind occurrence of ‘err mailbox isn't a valid mbox file’ error message is due to some problem in implementation procedure of IMAP, used by the server of email. In general, such issue affects IMAP servers that are known as DoveCot. Few amount of DoveCot servers describes that special folders can be enumerated, but fails with an error.

Like a part of IMAP protocol, an IMAP server describes whether a folder can be enumerated or not. MigrationWiz only extracts the folder contents, which are marked as enumerable. If marking is not done on any folder content then, MigrationWiz takes procedure as failed. Well, these folders may be any, which contain numerous amount of items within it.

In order to fix Dovecot error: "Mailbox isn’t a valid MBOX file", users need to perform anyone or both of the following activities:

  • Upgrade Your IMAP Server: In order to perform reporting of folders as enumerable or non-enumerable, keep your IMAP server upgraded.
  • Filter Failing Folders: In case, if IMAP server cannot be upgraded then, erase all the folders that are causing failure in migration procedure.


Mailbox files are most significant email files, which are created in multiple email applications like Apple Mail, Mutt Mail, Horde, etc. After understanding the various aspects of file, we have represented the entire information related to MBOX file along with the procedure to fix an error related to this file i.e., Mailbox isn’t a valid MBOX file.