1. Make sure that you have date and time set correctly on your phone.
  2. Check if there is new software available for your phone.
  3. Check your internet connection: Open the web browser and check if you can access your mail by entering the webmail address in your browser address field.
  4. Try using Wi-Fi instead of the mobile data. This is to bypass possible roaming or outgoing (SMTP) mail server limitation.
  5. Make sure that your mail inbox is not full. Move the emails from inbox to a temp folder. This is also to determine if there are corrupted emails causing synchronisation problems.
  6. Change your mail sync setting, which specifies in days how old items should be downloaded from the server. This can resolve the issue of not seeing an older mail items on your phone.
  7. Make sure that login approvals (two-step verification) are not enabled on your mail account. Refer to your mail provider to learn more about two-step verification.
  8. Delete and re-create your email profile on the phone. If this fails, switch your phone to flight mode and reboot. At this point you should delete your email profile, then turn off the flight mode and re-create your email.

The previous actions usually solve your problem. If it does not help, you may have to contact the service provider or compare the settings to another phone to determine whether the settings are correct.