What Apple Mail Export Mailbox actually produces

What lands on disk is a folder wearing a file's extension. Finder shows one item ending in .mbox. The real MBOX file that every other application wants is inside it. Point another mail client at the outer item and it fails. Point it at the file within and it works.

That one distinction accounts for most of the reports of Apple Mail exports failing to import anywhere else.

What Apple Mail Export Mailbox actually writes

Select a mailbox, choose Export Mailbox from the Mailbox menu and pick a destination. Mail creates something called Sent.mbox or whatever the mailbox was named.

That item is a package rather than a file. Apple's own documentation uses exactly that word, which is worth knowing when another application insists on being given a file. Finder hides the difference. The way through it is the contextual menu on the item itself.

The file named mbox is a genuine MBOX file in the standard sense. It is plain text, messages separated in the usual way, attachments carried inside the body as base64 rather than kept beside it. Anything that reads an MBOX file will read it.

This is not the same thing as the live store. Apple Mail keeps working mail as individual emlx files. A folder full of those is not an export. People who copy the store instead of exporting from it end up trying to import emlx files into applications that have never heard of them, which is a different failure with the same symptom.

Why the .mbox package causes so much trouble

Apple gave a folder the extension of a file format. The two behave nothing alike.

An application asking for an MBOX file expects a file. It gets a directory wearing the same extension and cannot make sense of it. Some refuse outright. Others appear to work while importing nothing. Both outcomes are widely reported and both look like a corrupt export rather than a wrong selection.

The fix is to go one level deeper. Control click the package and the menu offers Show Package Contents. Use the file named mbox inside it instead of the folder around it. Copy that file out somewhere ordinary first, since many applications will not browse into a package at all.

The reports of Thunderbird showing an imported folder tree with no messages are nearly all this. The structure came across because that is what was selected. The messages did not, because the file holding them never was.

What travels in an Apple Mail export and what stays behind

The read state is the one people notice. Those flags live in headers each client reads its own way, so mail arriving elsewhere can land entirely unread. The messages are complete. Only the marks on them are lost. The per message files the export reads from are EMLX, a format built for indexing rather than for moving.

What Export Mailbox will not do

Three limits, none of them documented anywhere prominent.

• No partial export. The command takes whole mailboxes. For a handful of messages, drag them from the message list into a Finder window instead. You get one EML file each.
• No overwrite. Run it twice on one mailbox and Mail numbers the second rather than replacing the first. Duplicates pile up quietly.
• No progress that means anything. A large mailbox takes far longer than the interface suggests. There is no completion notice worth trusting.

One thing it does do, despite claims otherwise, is handle several mailboxes in one go. Apple's documentation says select one or more. You get a package per mailbox. Anywhere telling you to export them one at a time is describing a restriction that is not there.

Re importing into Apple Mail

Going back the way you came is the one case where the package works as intended.

Import Mailboxes on the File menu offers a list of sources. The choice between two of them decides whether this works.

Apple frames the second option as the one for material arriving from a Windows or UNIX machine, which is why it goes unnoticed by people holding a file that came off a Mac. Pointing the Apple Mail source at a bare file or the mbox source at a package is the usual reason an import reports nothing to bring in. Whatever comes in lands in a mailbox called Import at the foot of the sidebar, not in the one it was taken from. Moving it into place is a separate job.

Watch for a reported size ceiling. Import failures on large mailboxes come up enough to plan around, with two gigabytes cited as the point where things start going wrong. Apple documents no such limit, so treat it as a reason to export in several mailboxes rather than one enormous one.

Re importing into Thunderbird or another client

Thunderbird is the common destination and the common source of complaints, almost all of which come back to the package.

Thunderbird has no native mbox import, which surprises people given that mbox is its own storage format. The route is the ImportExportTools NG add on, pointed at the extracted file rather than the package.

Folder hierarchy does not survive the trip. A single MBOX file holds one flat mailbox and carries nothing about nesting, so a tidy structure on the Mac arrives as a single heap. Exporting each mailbox separately is what preserves the shape, at the cost of doing it more times.

Deciding what to move and proving it arrived is a separate job from running the export. Destinations that are not mail clients need something different again. Getting mail into PDF, into a document system or into whatever format a business keeps records in means parsing the MBOX and writing something else. An MBOX converter for Mac does that without needing either application open.