It seems in Outlook 2011 SP1 (14.1), Microsoft re-added the .mbox export feature, which previously was deprecated from Entourage.
It is now possible to simply drag an Outlook 2011 folder from the list to your desktop, and have an importable Apple Mail .mbox.
The rest of the article is left for archiving purposes.  Thanks!
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Microsoft Outlook 2011 for Mac was supposed to be the most compatible release for Mac – but not necessarily the most open. Microsoft removed folder-dragging mbox exports in Outlook 2011, leaving only an export to a proprietary Outlook 2011 .olm file. This essentially locks the e-mail data into Outlook….almost.

Exporting Outlook 2011 Mail

In Outlook 2011, perform an Edit –> Select All in the folder or Inbox you wish to Export. With patience, click & hold the mouse until you see the number count come up on the cursor. Then move the cursor over a Finder folder window and release the mouse. Patiently wait while Outlook locks up & churns from exporting the e-mails. There will be no progress bar. This will fill the folder with .eml files of all the e-mails.
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Batch Convert & Import Microsoft .eml files into Apple Mail

1. Use registered eml-2-emlx to batch convert the files into an .emlx-based .mbox which Mail supports. Stephan should get back to you within a day if you pay with PayPal.
Download eml-2-emlx: http://aconcahua.com/eml2emlx/ 1.1.4 mirror: http://j.mp/ajnYg4
2. Quit Apple Mail & manually drop the converted emlx-based .mbox file(s) from eml-2-emlx into the user’s mailbox location (~/Library/Mail/Mailboxes). Apple Mail’s import function will NOT import the resulting .mbox files from eml-2-emlx.


3. Open Apple Mail & perform a Rebuild of the folder from within Apple Mail.

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Individually Importing .eml Files Into Apple Mail

Apple Mail has full support for it’s previous mail format standard, .emlx. Apple Mail does not have database support for .eml files – but supports reading & opening (looking at) them. Apple Mail has an internal converter for .eml to .emlx files if you wish to save a .eml file into a Apple Mail folder. When viewing an .eml file in Apple Mail, you can perform Message –> Move To –> * Folder and it will do so.
Navigating to the Mail folder location in the Finder (~/Library/Mail/Mailboxes/* Folder.mbox/Messages) will show your newly converted .emlx files. However, doing this in bulk would be highly tedious – and Automator doesn’t support move to actions in Mail:
It would also be technically possible to create a folder in Apple Mail, and then drop .emlx files into the Finder location of that folder to manually import existing .emlx files into Apple Mail. Afterwards in Apple Mail, selecing the mailbox and doing Mailbox –> Rebuild would then update the Mail folder with your newly inserted .emlx files.
* Note
This project was originally come across from trying to recover a corrupted 20 GB Entourage 2008 Mail Database. It was found that Outlook 2011 could successfully import the corrupted Entourage 2008 Database. From Outlook 2011, an export was necessary to convert the user to Apple Mail.