Common Issues Faced by MBOX Users

Similar to many other file formats, MBOX encounters various issues that require timely resolution to safeguard data integrity. One example is the need to reduce the size of an MBOX file as it grows larger.

Users may encounter various challenges when dealing with MBOX files, such as difficulties in opening files, compatibility issues with different platforms, and file corruption. While recent iterations of Thunderbird no longer impose a 4GB limit on MBOX folder sizes (unlike earlier versions), it's generally advisable to keep file sizes smaller. This precaution is due to the fact that the maximum allowable file size can also be influenced by the operating system's file system, and larger files may lead to abnormal behavior in certain email clients.

2 Less-Known Solutions to Deal with MBOX Oversizing

MBOX, alternatively referred to as the Berkeley format, serves as a common storage format for emails and is supported by a wide array of email client applications. These applications encompass popular ones such as AppleMail, WebMail, PocoMail, Gmail, Postbox, Entourage, Eudora, Opera, Netscape, Thunderbird, Spicebird, Evolution, PowerMail, Claws Mail, SeaMonkey, Sylpheed, GyazMail, Gnu, Mulberry, Sparrow, The Bat!, and many more.

Managing large MBOX files can pose a challenging endeavor. Nevertheless, there are two potential solutions to explore for reducing the size of these MBOX files:

Delete unnecessary emails and compact folders

The initial action one might consider taking to free up space is the following: Review your mailbox to identify unnecessary emails and remove them. Nevertheless, in numerous MBOX email clients, it is necessary to compact folders in order to recover the space that becomes available after deleting items, as this process is not performed automatically.

Split MBOX File using a Text Editor

Dividing your MBOX files using a text editor presents an effective approach to minimize mailbox size. You have the flexibility to employ various text editors, such as Notepad++, JujuEdit, EditPad Pro, EditPad Lite, EmEditor, HiEditor, HxD, ifhex, HexFiend, UltraEdit, VEDIT, VEDIT Pro64, and Vim, for this purpose. It's important to verify the compatibility of these editors with your operating system before proceeding. Once you've successfully installed your preferred text editor, proceed with the following sequence of steps:

  1. Duplicate the mbox file, creating two additional copies.
  2. Open one of these duplicate files using a text editor.
  3. Position your cursor in the middle of the file and search for the header labeled "Message ID," which contains a unique identifier for each email.
  4. Note: You may want to copy this ID separately to a notepad or notes app for future reference.
  5. Scroll down to the end of the message, delete all content below that point, and save the file.
  6. Now, open the second duplicate file and locate the unique Message ID where you initially split the MBOX file.
  7. Delete all messages above this Message ID and save the file.
Convert MBOX file to Outlook PST

If you find the previously mentioned methods unsatisfactory or seek an automated solution, consider converting your MBOX file into PST format and then splitting it into multiple segments. The Kernel MBOX to PST converter is a convenient tool that enables you to effortlessly convert all your MBOX files into PST format and divide the resulting file into multiple files according to your specific needs.

Once you've successfully transformed and divided the MBOX file into PST format, you can effortlessly access its data by opening the PST file using any version of Microsoft Outlook. This method proves to be the most effective solution for reducing the size of MBOX files.


MBOX users have several options for reducing the size of their MBOX files. To start, they can begin by deleting unwanted emails and then proceed to compact their folders. Alternatively, they can manually split their MBOX files using a text editor. If neither of these approaches proves effective, another excellent choice is to utilize the KernelApps tool to convert the MBOX file into the PST file format.