Once upon a time, I used Outlook Express and then moved onto Entourage. I used to save my sent and deleted email each year as individual files and then create a text index so I could search them using Sherlock. I had quite a few CDs corresponding to each year on my shelf. One day I decided that I'd like these all to be on my OS X box. After all, we have more disk space these days.
I created a special user called mailarchive and this user would hold all these messages.
The problem was: how do you convert these folders of individual mail files into corresponding mbox files which can then be imported into Mail. Now, if you only have a few hundred, you can drag them back into Entourage from the Finder, and then export them as an mbox file. However, if you have many thousands, you can't really do that because the Finder will go quite comatose if you drag 7,000 files and try and drag them into an Entourage folder. Here is how I solved the problem. It probably helps if you know a little Unix.
Here's what I did:
$ mv flip.osx ~/bin/flip
$ chmod +x ~/bin/flip
If you don't have a bin directory then you can do
$ mkdir ~/bin
If you want, you can of course also run flip from your Desktop directory in the Terminal. The next thing to do was run flip -u on each file. Because I had so many, I had to use xargs in the command line ... that is, I changed into the directory which contained all the .eml files. Assuming your eml files are in a directory on your Desktop called THEM you would do:
$ cd ~/Desktop/THEM
$ ls * | xargs flip -u
$ eml2mbox -s THEM ~/Desktop/WhateverYouWant.mbox
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040427005009722