Trouble transferring Mail folders to a new Mac

A friend of mine had a similar problem when migrating local mails from a POP mail account from Windows XP (Thunderbird) to a Mac (Mail.app).

This is what we did:

1. Create an IMAP account (we chose aol.com, gmail.com is another good choice).
2. Configure the new IMAP mail account in Thunderbird.
3. Drag and drop all mails from the local account to the IMAP account. This may take a while.
4. Configure both your current mail account and the IMAP account in Mail.app.
5. Copy the files back to your current mail account (again, drag and drop).

(You may consider keeping the IMAP mail address...)

This may be easier than troubleshooting why the import doesn't work properly.

To save all mails individually in the Finder, use a little trickery:

In Mail, select all messages you want and right-click + choose Forward as Attachment. This creates a new e-mail which you then send to yourself.

When the e-mail has arrived, bring it to the front and press Cmd-A to select all attachments in the e-mail. Click the Save button and choose Save All… from the drop-down menu, then select or create a folder in the file dialog.

The e-mails will be saved as individual .eml files, which you can quickview in the Finder or open in Mail or Thunderbird.

The downside is that the files are saved with the subject line as file name. So there is no date or sender to organize the files in the Finder. You'd have to pre-organize in Mail.