Mail for Mac OS X format is not the same as mbox files
Mail for Mac OS X format is not the same as mbox files

I think the title says it all, but the problem is bigger than that. The whole idea that the last 3 or 4 letters of a filename are an indication of underlying file format and structure is flawed. More than flawed, it’s wrong, but 3 decades of MS-DOS (yes, it’s still part of Windows) and its usability nightmare known as filename extensions is hard to overcome.

Interestingly, I don’t blame Microsoft for this particular confusion though, since it was Apple that broke the generally accepted, or de facto, standard in this case.

With the release of OS X, Apple introduced a new kind of file — or really a folder that acted and looked like a file to the user — called a package. The idea was that the insides of certain folders were only for system usage and should be hidden from users. For example, applications and all the various libraries and resource files and executables were packaged into a .app folder. To the end user, this .app folder looked and acted like a standard file and it could be double-clicked to launch the application. Early versions of Mac OS X even hid this package extension from the user, but to this day, to see the contents of a package, you have to “right-click” or “control-click” on the package and select “show package contents” to see what’s inside.

I have had 2 big problems with this new package construct:

  1. No other OS has this construct and writing cross-platform code to deal with it is more difficult than it should be.
  2. The Mac OS X file open/save dialogs do not hide the contents of the packages the same way the Finder does. Being that Mac OS X is otherwise very particular about consistency in its UI, I call this a bug.

Now, back to the topic of this post, Apple’s Mail application for Mac OS X uses “.mbox” as the package suffix for the folders that hold email message files. This is in direct conflict with the de facto standard of using “.mbox” as the file name extension for standard mbox files, and this creates incredible confusion for Mac and PC users alike when they are looking to convert their mail to or from mbox format or Mac OS X Mail format. In particular, Mac users looking to import mail into Mac OS X Mail get very confused with this and Apple should really do something about it. To make matters even more confusing, back with the release of Mac OS X 10.4, Apple removed the “package-ness” of the .mbox folders and now users can browse their contents freely. So, since these folders are no longer packages, why do they still need the .mbox name extension? Again, inconsistency.

So, to help deal with this confusion, I offer up these 4 rules of thumb:

  • Mac OS X Mail folders are not Standard mbox files.
  • Standard mbox files are not Mac OS X Mail files.
  • If you have a folder with a name that ends with “.mbox”, it MAY be a Mac OS X Mail folder, but it is DEFINITELY NOT a standard mbox file.
  • If you have a file with a name that ends with “.mbox”, it MAY be a Standard mbox file, but it is DEFINITELY NOT a Mac OS X Mail folder or file.

I hope this helps.

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3 thoughts on “Mac OS X Mail’s .mbox folders are not standard mbox files

  • January 26, 2009 at 2:41 pm
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    I recently tried the demo version of EmailAlchemy and found it worked great to find and recover email from the Entourage Cache. However, the program recovers the files with a “.mailbox” extension.

    Entourage does not recognize this as a file that can dragged and drop to import the email back into Entourage. If the file is renamed to end in “.mbox” then it can be dragged and dropped. I don’t know if this is related to the Apple mail inconsistency but for the sake of easier import I recommend the recovered cached email messages be named with the “.mbox” file extension.

  • January 26, 2009 at 7:18 pm
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    Yes, the default may be .mailbox, but you can change it to .mbox, .mbx or whatever you need in Emailchemy’s preferences. Also, for easy Entourage import you can convert to the Entourage Archive format (.rge file). An .rge file is can contain an entire hierarchy of mailbox folders that can be imported in one step.

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