Hi ozdazza,
I have managed to solve the problem and move all my emails from my old mac running 10.5.8 to my new mac running 10.7.2 with the following steps. Please note that in order for these to work, you need to have copy of your old email accounts either at your new mac desktop or in your time machine (or any external HD connected to your new mac)
1- In my new mac, I created email accounts similar to the ones in my old mac (names and setting),
2- I copied my old emails (one email account at a time so I do not get confused) and placed it on my new mac desktop. Please note that inbox messages in OS 10.5.X are stored in user/library/mail/pop.file/inbox.mbox. And outbox messages are stores in user/library/mail/pop.file/outbox.mbox and so forth. For copying messages in your inbox the file you should copy to your desktop is the inbox.mbox NOT the mail file or NOT the pop file.
3- Open your mail app in your new mac and chose “import mailboxes” under file menu.
4- Chose import data from “Apple Mail” which is the first choice.
5- Chose the inbox.mbox file to be imported.
6- After the importing process finish, you will see all your messages in a new folder called “Import” in your mailboxes column within your new mail app.
7- Now click on that import box to display all your messages in the middle column in your email app.
8- Highlight all the messages by clicking comm+A
9- Drag all the highlighted messages to your newly created inbox.
10- After the drag&drop process finished, you can delete the import box.
Above process can be done with the sent.mbox as well, but in such process you use the sent.mbox for old email account and drag its messages (step 9) to the sent file in your new email account box.
I had managed to move or transfer 3 emails account each one with 2 boxes (inbox and sentbox) with a total size of over 8GB in only less than 10 minutes.
The key problem in all my previous import attempts is that I was importing the wrong file and trying to place it in the wrong place.
I hope above process works well for you as it did for me.