1. Check your mail settings and make sure you are not set to automatically mark emails as unread.
2. Check that the emails have been transferred from your device to the server and not just appearing as sent and not received.
3. Verify that you have updated all your mail app/client settings and make sure unnecessary software processes are not running.4. Make sure your mail server settings are updated and perform a manual check for any unknown accounts.
5. Try reseting the client software and restarting your device.
6. Last but not least, contact your mail service provider and see if they can assist.