Computers and Technology

Discover the Easy Ways to Fix “Mailbox is not a valid MBOX file” Error

Full Form of MBOX File Format – Mail Box File

Are you a user of Thunderbird, Eudora, SeaMonkey, Apple Mail, or PocoMail? If yes, you may be using the MBOX file extension to save emails and attachments. MBOX is one of the most common file extensions supporting over 30 email clients operating on Windows, Mac, and Linux. The full form of the file extension is Mail Box File. It has been used by several email client users. Small businesses like to use MBOX to avoid data sharing issues.

MBOX Structure & Location of the File

One can easily differentiate an MBOX file from other files because each MBOX file carries the extension name in the file name like a file.mbox. The extension helps to identify MBOX files easily. You can identify the file with its icon. MBOX supports desktop-based email clients. It stores data in plain text format. The file comprises emails in the form of 7-bit ASCII text and the rest components like CC, BCC, To, From, and attachments are stored in encoded form.

MBOX is so popular among email client users because of its feasibility to support multiple email clients. If you want to switch from one MBOX supporting email client to another MBOX supporting email client, you don’t need to think much. Moreover, MBOX supports Windows, Mac, and Linux. Therefore, conversion from one operating system to another is not an issue if you are using an MBOX support email tool. For example, users of Thunderbird for Windows can easily switch to Thunderbird for Mac. It doesn’t need data conversion. The same MBOX file can open in Mac and MBOX. Doesn’t it sound great?

All about types of MBOX mailbox files

MBOX File Structure: Do you know in a string of concatenated email messages, all emails are stored in a long plain text file. The starting point of a message indicates in From with a white space character and the ending point with a blank link. MBOX files have some variations that you must know.

Mboxo: The file format is a bit different from the real MailBox file as it holds a constant string like “???@???” in the sender’s mail address,

Mboxrd: It is an extended form of Mboxo file. In this file format, the saved message is displayed at the initial point in the header or the body of the content.

Mboxcl: The file extension is supported by the Unix ‘System V’ mailboxes

Mboxcl2: This is the last variation of MBOX that is an upgraded version of Mboxcl file. To point out the email separator, it doesn’t scan the ‘From’ lines.

Location of MBOX File: It is difficult to determine the location of an MBOX file. It is based on the email client you are using. It is also based on the operating system you are using. In other words, the location of the MBOX file is not well defined.

For Mac Users MBOX File Location:

Here are the steps to locate an MBOX file on Mac OS X Mail Edition 1:

  1. First launch the Finder and then press the Go >> Go to folder button
  2. Type ~/Library/Mail/V2 for Mac OS X Mail 5-8 or ~/Library/Mail/V3 for Mail to find the location.

Location of MBOX File for Windows users:

The following location is for those who are using Thunderbird for Windows.

  1. Click on the Run window and then execute %APPDATA%
  2. After that, click on the Thunderbird folder and then the Profiles folder
  3. You can able to see each folder.

Some Problems With MBOX File Format

MBOX is not different from other email file extensions. It is also prone to corruption. Users of MBOX complain of some problems. The article has picked up some common problems faced by MBOX users.

  • Fail to access emails
  • File size issue
  • Emails are not uploading
  • Mailbox isn’t a valid MBOX file

These are some common issues with MBOX files. The article shares how to fix the valid MBOX file error.

Steps to Troubleshoot “Mailbox isn’t a valid MBOX file” Error

MBOX files have the issue because of the implementation procedure of IMAP the email server is using. In other words, the issue can cause problems with your IMAP servers. They are known as DoveCot servers.

When the problem is developed, the server tells whether a folder is enumerated or not. MigrationWiz helps to extract the content of the folder that is marked enumerable. If the folder is not marked, MigrationWiz fails to do so. These folders contain a large number of items.

Users can fix the Dovecot error with the help of these two activities.

  • Upgrade the IMAP Server: The first measure you can opt for is to upgrade the IMAP server so that you can report folders as enumerable or non-enumerable.
  • Filter Failing Folders: The second process is suitable when the first process doesn’t work or seems ineffective. Under this process, delete all the folders resulting in the failure of the conversion process.

How about MBOX to PST Conversion? MBOX supports free tools. These are basic tools for email management. MS Outlook is an advanced utility to manage emails, contacts, notes, journals, and several other sub-folders systematically. For the conversion of MBOX files, the best way is a third-party conversion program. Use a professional Softaken MBOX to PST Converter tool that gives an assurance of accurate data conversion. Make sure the interface of the application is interactive.

Softaken MBOX to PST Converter


The article tells a lot about the MBOX file extension and why people are using the MBOX file format. It also reveals the common problem with the mailbox is not a valid MBOX file. MBOX is a common file extension. Users can face problems with MBOX.

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