@Heidihi: Please can you confirm the model number and current Android version of the device you're accessing your emails on? This can be found in Settings > About phone/tablet. Furthermore, which app are you using to sync your emails to the device? Have you tried signing in to the account via a browser app (Samsung Internet or Chrome) to see if your inbox is populating as expected?
In the meantime, to eliminate the possibility of the issue being storage related, please try deleting some emails, starting with your Sent folder and Junk/Spam folder, to see if this helps.
