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Mailbox isn’t a Valid MBOX File – Solving a Frequent Error

Published By Siddharth
Aswin Vijayan
Approved By Aswin Vijayan
Published On January 24th, 2024
Reading Time 6 Minutes Reading

Are you among those who are facing the “Mailbox isn’t a valid MBOX file” error? Have you tried to resolve this issue? Or do you want to know a quick trick to rectify this problem? If your answer to all the questions is yes, then you have landed in the right place. Here, in this blog, we will enlighten users about how to fix an error syncing inbox failed mailbox isn’t a valid MBOX file manually. We will also take a look at other MBOX errors.

As we, all know with the increasing technology, emails are one of the major factors for performing cross-communication. Mozilla Thunderbird is among all other email applications, which is used for this purpose. It stores the data in MBOX format. It is a versatile format to use and is supported by various email clients. There are certain errors that are associated with the MBOX format. A few of them are:

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

Here, we are discussing the last error “Mailbox is not a valid MBOX file”.

Reason Behind the Error Mailbox Isn’t a Valid MBOX File Message

We have further divided the sections in two parts based on the general occurrence of the error.

Error In DoveCot –

A bug in the IMAP implementation often causes the “Mailbox isn’t a valid MBOX file” error. Especially when used by IMAP servers like Dovecot. You will find most of these errors related to Dovecot on the Internet, while others occur in Thunderbird, Roundcube, etc. Some DoveCot servers specify that it is possible to enumerate special folders, but the user or tools fails with this error when trying to retrieve content. In our case, some mailboxes that contain the “Mail/dovecot-uidlist” system folder return this error.

Some tools like MigrationWiz only access folders’ content that are marked as enumerable. If such a folder is not accessible then the “Mailbox isn’t a valid MBOX file” error occurs.

Some errors occur due to a line break during the MBOX synchronization process. You can understand this by taking a look at some of the problems people had:

  1. https://forum.howtoforge.com/threads/dovecot-error-mailbox-isnt-a-valid-mbox-file.62792/
  2. https://stackoverflow.com/questions/63880514/roundcube-mailbox-isnt-a-valid-mbox-file
  3. https://blog.csdn.net/weixin_33739646/article/details/91651919

Error In Mozilla Thunderbird –

MBOX is a versatile file format therefore used by several organizations for business continuity. The supporting email application of this format is Mozilla Thunderbird. Sometimes, users accessing inbox in Thunderbird may get an error message like “Mailbox is not a valid MBOX file.” One of the major reasons behind this issue is the corruption in the MBOX file. It can be due to improper shutdown of an email application, virus attack, power failure, etc. Let us take a query to understand the issue faced by a user:

Query – “While accessing Mozilla Thunderbird, suddenly an error message came to my screen as “Mailbox is not a valid MBOX file.” It was the first time I was getting this error. I have tried to rectify it by using some manual solutions, but all my efforts go in vain. Tired and dejected, I have decided to take the guidance on this. Therefore, now I want to know a relevant trick to fix error syncing inbox failed mailbox isn’t a valid MBOX file. Please help me out if anyone knows the solution.”

Best Possible Solutions for “Mailbox isn’t a Valid MBOX File” Error

Similar to the reasons, the solutions are also divided into two sections.

Solution for Dovecot –

If it’s a problem with one of the tools like MigrationWiz, then it can be solved by doing the following:

  1. Upgrading IMAP Server – If possible, upgrade the IMAP server that correctly reports folders as enumerable and unenumerable.
  2. Filter Failed Folders – If your IMAP server can’t be updated, you can filter (to exclude) the folders causing this error. Then, later perform the tasks manually on these folders.

If it’s a problem with synchronization, then, in most cases, look for the leading “line break” and remove it in the /var/spool/mail/user file.

Not a solution, but if you have the option consider switching to other formats like Maildir and most of these issues with MBOX file will be gone.

Solution for Mozilla Thunderbird –

This problem can be rectified by enabling IMAP logging. It can be done with the help of a logging proxy, which is an additional program to log a specific connection. You need to follow these steps to perform this task:

  • Firstly, log a server type
  • You do not view the state of Mozilla Thunderbird’s connection. It simplifies the log, but it may result in some issues that are more difficult to discover
  • You will read the raw data without any explanatory information
  • Moreover, you will view the information on authentication. Nevertheless, to investigate secure authentication you need additional tools
  • You cannot log a connection, which is secure.

For the configuration of proxy, you need to change Mozilla Thunderbird’s settings or you may change proxy settings to indicate localhost as a proxy server. By modifying the settings in Thunderbird, you may risk changing the nature of the issues that you are examining. If it is feasible, examine the proxy configuration with a running server before using it with the server you are examining.

To modify Mozilla Thunderbird server’s settings, point Mozilla at the server localhost & point the logging proxy at the real server.

  • Now, to change proxy settings Tools>>Options>>General>>Connection Settings
  • Then, select “Manual proxy configuration” and set HTTP proxy as localhost
  • After this, mark the box “Use this proxy server for all protocols”
  • Finally, select the port; you have configured in the proxy.

Now, this method is quite tricky and may leave users in a confusing state. In addition, it is not a foolproof solution for the rectification of this error. This is the reason behind the need for users to look for a method that is relevant and assures guaranteed results. With this, they can easily repair corrupted or damaged MBOX files. Moreover, it is important to opt for the right software.

If you ever want to convert MBOX files to a better or more accessible file format like PST, PDF, EML, HTML, MSG, you can opt for the best MBOX to PST Converter software out there. I would recommend as it has all the mentioned export formats and is a trusted brand in the industry.

The Bottom Line

Email servers may return the “Mailbox isn’t a valid MBOX file” error due to buggy IMAP implementation. Of course, it can also be due to corruption, especially in the case of Thunderbird MBOX files. Considering this, we have come up with manual solutions to rectify this error.  Hopefully, it will get the job done.