This is causing me some pain, but it's a one-time kind of pain. I hope.

My wife is migrating from Windows XP to a Mac. Her email used to live in Thunderbird. I'm trying to migrate that to Apple Mail. For all but three folders, everything imported great. The three that failed were also the three largest folders, with the biggest clocking in around 1.5GB. Any of these would cause the importer to crash.

Fine. I cobbled together a Python program to split them into files of around 100MB a piece. I then tried to import these separately. Apple Mail was clearly scanning the files, as it was listing subject lines as it worked. At the end, though, I got folders of the appropriate names but no messages in any of those folders.

I installed Thunderbird, and it has no trouble whatsoever opening those files. It generated the accelerator files (.msf). All good, confirming my Python program didn't screw anything up. I imported again into Apple Mail. Again, zero contents. I asked Thunderbird to "compress" the folders (thus rewriting them and cleaning them up). Again, Apple Mail imported zero contents. Zooming down into Apple Mail's file store, I can see the Folder.mbox directories, but all with zero contents, versus Apple's normal one-file-per-message junk.

Any ideas on how to proceed?

and Thunderbird on OS X is deficient how?

Use Thunderbird?

Seriously. Does she have any experience with Mail.app? I hate it.

That said, does Mail.app have an auto-importer when started without any configuration? Maybe try getting everything into Thunderbird on the Mac and see if Mail.app will import it directly?

To be more direct I should say that we have only had problems with Apple's Mail app and generally try to set our OS X customers up to use TBird.

Fair points about Thunderbird. The main benefit of Mail.app is that it integrates nicely with Spotlight, making it easy to search your own mail. Also, Thunderbird's use of Berkeley mbox means things can get slow and behave strange as the box size gets into the gigabytes.

At any rate, the problem turned out to be that Mail.app's importer fails to understand deleted messages (which are still in the file, but marked deleted). Asking Thunderbird to clean-up the files seemed to fix the problem and everything is now properly imported.