To solve this problem, you may need follow these steps:
1. Open settings
2. Under Connection section, you may find Data Usage
3. In Data Usage page, touch the menu botton and tick Auto sync data
Then, you emails now can sync automatically.
1. Open settings
2. Under Connection section, you may find Data Usage
3. In Data Usage page, touch the menu botton and tick Auto sync data
Then, you emails now can sync automatically.