The topic of this post is : Fixing hidden Thunderbird emails.

If all of a sudden an entire hive of emails are hidden to you when using Mozilla Thunderbird, perhaps it's because they are internally marked as deleted even though you did no such thing. Fortunately, this is a situation that can be fixed with a proper... text editor !

First of all a little bit of context. In Thunderbird, mails for each account are stored on a per file per folder basis. In other words, your inbox is stored in a single file, whose name is the name of the folder and you can find those folders in your personal Windows files here : c:/users/<username>/AppData/Roaming/Thunderbird/Profiles/<profile_name>/

If your account is served by a POP mail account, it's in the Mail sub-folder. ImapMail for an IMAP-based mail account. There you find an Inbox file, which is the collection of your inbox emails stored as plain-text. This plain-text is called the MBOX file format, and you'll find reference for MBOX with Google, complete with a lot of useless discussions out there. Being plain-text means you can fix a "corrupt" file, as long as you understand the header marking the beginning of an email, and marking further down the stream the end of the current email.

Each email comes with a header such as X-Mozilla-Status: 0009. Number 9 marks a deleted email. So it's easy to guess that the next thing to do, to unhide those emails is to find/replace all such occurence with X-Mozilla-Status: 0001.

Of course, if you've been accumulating emails for a while, chances are your Inbox file will be bigger than 1 GB, in fact possibly bigger than 10 GB. A lot of text editors can't edit this thing because they haven't been properly designed, so you won't open a large Inbox file with say Notepad or something equivalent. First you need a 64-bit version of it, in order to bypass the 2 GB per process limit, and you need a text editor which is designed to edit large files, possibly without loading the file in memory at all. UltraEdit, which comes with a 30-day trial, is such candidate for this. So fire it up off internet and soon you'll be editing large inbox files, only to find that next time Thunderbird is started, all your emails have reappeared and the problem is fixed !!