Tips & Tricks

Fix Corrupt & Oversized MBOX File in Thunderbird Email Client

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With continuous flow of emails, the MBOX file in Thunderbird gets oversized after a certain point of time. This oversized file may turns corrupt or create performance issues with the application. In this post, we’ll discuss the ways on ‘how to reduce the size of MBOX file in Thunderbird’ and ‘what to do when the MBOX file turns corrupt’.

Fix Corrupt & Oversized MBOX File in Thunderbird Email Client

Thunderbird, developed by Mozilla Foundation, is an open source and free desktop-based email client. Like other desktop-based email clients, Mozilla Thunderbird allows users to set up various email accounts by using POP and IMAP. The email application is preferred by many users as it provides various useful options to easily manage emails, for creating contacts, tasks and calendar, and several others.

When you configure an email account in Mozilla Thunderbird, the mailbox data is locally stored on your system in the MBOX file format.In MBOX files, all the email items and related data are saved in a single text file. As you continuously receive emails that also include attachments, the size of the MBOX file also gets increased with time, resulting in an oversized file.

When the MBOX file gets large or oversized, you may start facing performance issues with your Thunderbird. One of the major issues that may arise from an oversized MBOX file is the corruption. However, the MBOX file may also get corrupt due to improper system shutdown, hardware failure, virus or malware attack, crashing of the email application, etc.

In case you’re facing performance issues with your Thunderbird application due to oversized MBOX file or the file gets corrupt, we’ve discussed below some ways on how you can deal with an oversized and corrupt MBOX file in Thunderbird.

How to Reduce the Size of MBOX File?

A large or oversized MBOX file may lead to various issues, therefore, it is advisable to reduce the MBOX file size. To reduce the file size, follow the given methods:

  1. Delete Emails

To reduce the size of MBOX file, the best approach is to delete the older and unnecessary emails. You can find many emails containing messages related to newsletters, manuals, advertisements, or other promotional content, which are unnecessary or not required. These emails considerably increases the mailbox file size.Therefore, deleting such unnecessary emails helps to reduce the size of your MBOX file.

  1. Compact the File

After you’re done with deleting unnecessary emails, you can also compact the folders so that the space generated by deleting the messages can be reclaimed.

  1. Split MBOX Files

You can also split the MBOX file by separating certain emails from the file and creating a new MBOX file,which contains the message information. This can be easily done by cutting and pasting the details from the desired emails in the MBOX file to a new mailbox file. As MBOX files are text-based files, you can easily identify and separate the emails based on the unique Message ID. You can also use a text editor application such as Notepad++, Edit Pad, etc. to split the MBOX file.

TIP ! If you don’t want to delete or split the oversized MBOX file, back up the complete file by converting it to PST format. For this, you can use a specialized Stellar Converter for MBOX that retains all the mailbox items intact after conversion.You can then import this converted PST into any Outlook account to access your MBOX file data.

What to do when the MBOX File gets Corrupt?

If the MBOX file gets corrupt due to increasingly large size or any other reason, you can follow the given methods to restore and access your Thunderbird emails:

  1. Synchronizing of Thunderbird by using IMAP Protocol

Thunderbird email client offers an option to synchronize user mailbox data with web-based email clients by using IMAP protocol. In case you’re not able to access the mailbox in Thunderbird, you can synchronize the mailbox with other email client such as Gmail to restore the MBOX file data.

  1. Re-install Thunderbird and Restore the MBOX File

If you can’t access your Thunderbird emails due to corrupt MBOX file, you can also reinstall the Thunderbird application and restore the corrupt MBOX file. For this, take a backup of the corrupted MBOX file in a folder on a separate location and then uninstall the email client.After uninstalling the application, install it again and move the backup MBOX file folder back to the original location. Then launch the Thunderbird to check if you’re able to access the MBOX file.

  1. Convert Emails to EML Format

In some cases, when the user is able to open the Thunderbird application, converting emails to other file format also helps in accessing the emails.You can convert the Thunderbird emails to EML format and then import them into other email client such as Outlook for Windows, which supports EML file format. To convert the emails to EML file, you can just ‘drag and drop’ the emails from the mailbox to the desktop or any other folder/location on the system.This way the emails will be automatically saved with .eml file extension. Now copy these emails to another email client to access your emails.


To Conclude

It is obvious that large-sized data files are difficult to manage and prone to various errors and issues. The same is the case with Mozilla Thunderbird’s MBOX file, which when gets oversized may turn corrupt or hamper the functionality of the application.Therefore, it advisable to check the MBOX file size from time-to-time. You can follow the methods given in this post to reduce MBOX file size in Thunderbird. If in case the MBOX file gets corrupt, you can use the different ways mentioned in this post to access and restore your mailbox.

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