Corruption or having oversized Files in Thunderbird Email client is very complex problem. To overcome with this issue user prefer to use manual methods so that they can repair their files or make file in sized form. Before telling what would be the manual and automated process to make your file free from corruption and oversized I will explain what are the main causes that lead you facing this problem along with this I will explain how thunderbird manages emails and if you are facing this problem then I will explain how to overcome it.

Causes for Corruption and Oversize MBOX File in Thunderbird

Corruption is a very common problem. Nowadays many people are facing this problem that they have corrupted or damaged file. And they are looking for best solution to repair their file. So, here I will discuss what make an MBOX file corrupted and oversized.

  1. Virus and other malicious attack may corrupt your MBOX file. Once the virus entered in system it will infect your all MBOX files and user can lost his data.
  2. Some time it happens that user deleted his emails but that emails still visible in file. Those deleted and new emails occupy space which mikes MBOX file oversized.
  3. Another problem of getting file oversized is many times one email appears many times on the same file and occupies space which makes the MBOX file oversized.
  4. An email get corrupted when emails arrived but do not visible in the file this is due to the file of MBOX emails is now corrupted.

How Thunderbird Email App Manages MBOX File?

Thunderbird is an open-source email or chat application used for managing the MBOX file format email items. This email app stores email messages in a file by utilizing the MBOX format. Thunderbird uses .msf extension file to cache information associated with the file list. Along with this, it also creates the index file to map the file names systematically. Apart from this, you can identify your inbox file in the MBOX file as inbox.

As you are already familiar that the MBOX file stores each and every message in the order where latest email message is saved at the end of the previous message. Furthermore, each email message has From_Line and blank line before and after a message, respectively. The Line, described in the preceding statement, starts with From, space, time & date. Thunderbird email apps also use these characters and definitions along with the X-Mozilla-Status: and X-Mozilla-Status2: headers. These two headers are responsible for explaining the current status of email messages, which are determined by predefined numbers and flags such as MSG_FLAG_READ, MSG_FLAG_REPLIED, MSG_FLAG_PRIORITIES, MSG_FLAG_NEW, and MSG_FLAG_TEMPLATE.

How Corruption Originates in Thunderbird MBOX Files?

As email messages get accumulated in the MBOX file file, Thunderbird email app is unable to track all email content because of lack in clarity, which is required to define the beginning and ending of each email message. Hence, in such cases, Thunderbird starts deleting email messages accidentally due to active Junk settings. Because of this, discrepancies are created in the MBOX file, which leads to the corruption of MBOX email items. However, you can compact the large-sized MBOX files using the Thunderbird application to implement the precautionary measure for avoiding any corruption. But, the process to recover corrupt MBOX files cannot be handled by the Thunderbird application itself.


MBOX to PST Converter software is the best conversion software to convert MBOX files from more than 30 types of email clients to PST format. After converting the files, you can access the files in Outlook completely. You can choose the entire mailboxes of user accounts and migrate them easily. The software’s advanced filter options make it easier to select only the essential files and leave the rest. The data is protected throughout the conversion process.

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