Apple Mail's .mbox Is a Folder, Not a File

When Apple Mail exports a mailbox it writes a folder named Inbox.mbox, and inside it a file literally called mbox with no extension. That file is standard MBOX — the folder around it is Apple’s packaging, not part of the format.

This trips up a lot of people, and in a specific way: the export looks like it worked, the reader refuses it, and you conclude the file is broken. It isn’t.

What is actually in the package

Right-click the .mbox in Finder and choose Show Package Contents, or open it in Terminal, and you get something like:

Inbox.mbox/
  mbox              ← your mail, standard MBOX
  Info.plist        ← Apple's metadata
  table_of_contents ← index, older versions only

Only the first one matters. Info.plist is a few lines Apple uses to name and identify the mailbox; the table of contents is an index some versions write and none of them need to read back.

If your export has subfolders, each is its own .mbox package nested inside, and each has its own mbox file. A mailbox with ten subfolders is ten packages, not one big file.

Why Apple did it this way

Because Apple Mail does not store your mail as MBOX at all. Internally it uses EMLX — one file per message, in ~/Library/Mail/, with the message and a bit of Apple metadata in each.

MBOX only shows up at the door, on the way out, because it is the format everything else can read. And rather than emit a bare file, Apple wrapped it in the packaged-folder convention it uses everywhere in macOS, where a thing that looks like a document is a directory underneath. Consistent with the rest of the system; confusing the moment the file leaves it.

Opening it

Point Mbox Viewer at the .mbox package and it resolves it for you: it finds the mbox file inside, opens that, and names the mailbox after the folder, so you see Inbox.mbox in the sidebar rather than a nameless mbox. Drag the folder onto the window, or use Open. There is a full guide if you want the steps.

If a tool you are using insists on a file, go inside the package and hand it the mbox file directly. Copy it out first and give the copy a real name — inbox-2019.mbox — because a folder full of files all called mbox is a filing disaster waiting six months to happen.

On Windows it is worse, and simpler

Copy an Apple Mail export to a PC and Windows shows you a plain folder called Inbox.mbox, because the package convention means nothing there. That is actually helpful: you can see the mbox file straight away and open it, with no Show Package Contents step. The Windows guide covers it.

One thing to watch when copying: keep the whole folder, not just the file you can see. If the export has nested mailboxes, the ones you leave behind are gone from the copy.

Exporting it in the first place

In Mail, select the mailbox and choose Mailbox → Export Mailbox…, then pick a destination folder. You get one .mbox package per selected mailbox.

Worth doing before you need it. An Apple Mail export is one of the few backups that will still open in fifteen years without Apple’s involvement, and it takes a minute. It keeps working after the account it came from is closed, which is rather the point of it.

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Related: EMLX in the glossary, and what to do when double-clicking does nothing.