Thanks, but no help. I set it to 10000 and it do resync, with some more files back, but many still not being sync back, while the token is now back to (and greater than the original number 525126). I am now setting it to 1 to see how it goes (just confirm it doesn't sync all back while I typed).
The dummy way I just discover that will work is to mark all mail as unread in the folder, then it will sync over. In Outlook, mark the folder back to read. This work, but create a lot of work in order to mark all subfolders as unread (there seems no option to mark folder as read similiar to Outlook).
Btw, is there are message limit for the sync? While doing the above, seems like to sync stop after hitting a subfolder with 558 messages. Everything before that folder is okay, everything after that folder is blank (need to do the above to recover).