As I've lost almost all of the folders/sub-folders and emails except those in the Inbox/Sent folders, I tried to recover them from the Google email server site. But, when I tried to download the emails to my Thunderbird Portable, I got the following error message.
!!! The folder Inbox is full, and can't hold any more messages. To make room for more messages, delete any old or unwanted mail and compact the folder!!!
The Inbox root has 1854 emails, and the Sent root has 737 emails, which are not too many in my experience. In fact, since I've lost almost entire emails, there should be a more than enough space to receive the backed-up emails from the Google server. Why do I have to compact the folder?