This is a quick step-by-step guide to updating the IMAP path prefix in Mac Mail (OS X), to correctly route your mail program to show all folders.
How to update the IMAP path prefix in Mac Mail
Open Mac Mail
Firstly, you'll need to open Mac Mail by clicking on the Mail icon, in your dock.
Choose 'Mail > Preferences'
Once Mac Mail has opened, choose 'Mail' from the main menu, then from the drop down menu choose 'Preferences'.
You will be presented with dialog box. Please click on the option labelled 'Add Account'.
You will then be presented with a page related to your email accounts. If you have more than one email account, choose the account you wish to edit from the list of the left hand side. Then click on the option labelled 'Advanced'.
Edit 'IMAP Path Prefix'
Scroll down to the field labelled 'IMAP Path Prefix'. In the text field, enter "/".
Save Your Changes
When you have made these changes, close the dialog box. If you are prompted to save your changes, click on the 'Save' button. Your mail setup is complete. You may need to restart your mail client for the new settings to take effect. Once completed, you should be able to see all files that were present on the old mail system.