solving the mystery of the failed thunderbird import
you have an mbox file, you have thunderbird, and you have the "importexporttools ng" add-on. it should be a straightforward process, but your import keeps failing. either nothing happens, you get a vague error, or the process seems to complete but no emails appear. this experience is common, but the reasons for it are often hidden and poorly documented. this article will shed light on the five most frequent culprits behind a failed mbox import in thunderbird.
reason 1: your mbox file is too big (>4gb)
this is perhaps the most frustrating and insidious reason for failure. the importexporttools ng add-on has a known, long-standing issue where it cannot properly handle mbox files larger than 4gb. what makes it so difficult to diagnose is that it often fails silently. it doesn't pop up an error message saying "file too large." it simply skips the file, leading you to believe the import worked when, in fact, nothing was imported at all. if you're working with a google takeout archive or a long-term backup, your file is very likely over this limit.
reason 2: your file has the "wrong" name (no .mbox extension)
many email clients, including thunderbird itself, create mbox files without a `.mbox` extension. for example, your main inbox file is probably just named `inbox`. however, the import tool can be finicky and may be configured to only look for files that explicitly end in `.mbox`. when its search comes up empty, it can't proceed. users who find their `inbox` file and try to import it are often met with failure simply because of this missing extension.
reason 3: you're missing the required add-on
this might seem basic, but it trips up many new users. thunderbird does not have a native, built-in function to import mbox files. you *must* install the third-party "importexporttools ng" add-on. without it, the option simply doesn't exist in the menus, leaving users confused as to why their version of thunderbird seems to be missing a key feature.
reason 4: no "local folders" destination is available
the import tool needs a place to put the imported emails. it typically defaults to a "local folders" account within thunderbird. on a brand new, clean installation of thunderbird where no email accounts have been set up, this "local folders" destination may not be visible or properly initialized. this can cause the import process to fail because the add-on has nowhere to deliver the mail.
reason 5: the mbox file itself is corrupt
finally, the issue might be with the file itself. because of the simple, sequential nature of the mbox format, even a small amount of data corruption—a misplaced character or a faulty "from " separator—can make the entire file unreadable to a strict parser like the one used by thunderbird and its add-on. the import will fail because the file no longer adheres to the expected structure.
solution #0: bypass all five problems at once
before you start diagnosing these five potential issues, consider a simpler path. a tool designed specifically for viewing—not importing—can bypass these problems entirely. the mbox viewer chrome extension:
- is built to handle files far larger than 4gb.
- is extension-agnostic; it doesn't care if your file is named `inbox` or `inbox.mbox`.
- requires no add-ons.
- doesn't need a "local folders" structure because it's not importing anything.
- has a more tolerant parser that can often read files with minor corruption that would cause other tools to fail.
by using a dedicated viewer, you sidestep the entire fragile import process and get straight to your data.