If you are using Office 365 and cannot see your mail folders in Postbox, please do the following:
1) Navigate to Preferences | Options > Accounts > your Office 365 account > Advanced button.
2) De-select the option for "Show only subscribed folders."
3) Select the option for "Server supports folders that contain sub-folders and messages."
4) Close the preferences panel and restart Postbox.
5) Go to the File menu, select Subscribe... then select your account in the pulldown menu at the top.
6) Click the refresh button.
7) If there are missing sub-folders, collapse and then expand the parent folder, and the sub-folder should then display.