Yes, some time around (I believe) Mail 6.0 Apple changed the 'mbox' format created during export. As for Thunderbird if I recall correctly you have to go 'inside' of the.mbox file and rename the mbox. Sounds a bit complicated so here is a step by step example: Let's say you have an 'Inbox.mbox' from Apple mail. This is actually not a file but a container, a folder. Double-click the 'Inbox.mbox' or right-click and select open (whichever works). Inside you will find 3 files but the only relevant one is called just 'mbox'.

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Rename this file to 'Inbox'. You should now have: Inbox.mbox > Inbox Inbox.mbox > info.plist Inbox.mbox > table_of_contents If you import this 'Inbox.mbox' into Thunderbird it should have all the mails inside. If that doesn't work then drag & drop the renamed 'Inbox' file to the desktop, rename it again to 'Inbox.mbox' and try importing. If I'm not mistaken one of those should do the trick. Thunderbird uses MBOX for storage, so you really should have no difficulty importing the folders, unless there's something funny about the MBOX files created by your Apple Mail.

Currently, it is causing the following error message to show: A COM EXCEPTION HAS OCCURRED. While we work with Microsoft to understand and come to a solution, we recommend the following: El Capitain users should stay on or roll back to Word version 15.21.1 Yosemite users, please continue to use Word version 15.20 Instructions on how to roll back to a prior version of Word from both El Capitan as well as Yosemite are here: Please watch this thread, which will be updated as we learn more. Microsoft word 2016 for mac messed up.

Do the following test. Take one of the MBOX files and place it along with Thunderbirds other mbox files (you can find the location of TB's storage from the Preferences. It's listed in the Accounts section in the 'Servers' panel). Restart TB and see if the folder appears in the list and its contents displayed properly (it may take a few seconds for TB to do this as it needs to build a new index for the folder). If it works, you can import the rest of the folders in the same way. If not, the files exported by Apple Mail are not true MBOX (or are corrupt), and you need to export them again.

There are other ways of migrating your mail (e.g. By using IMAP), so if this doesn't work, get back to us and we'll suggest other steps.