1. Check if the email address is valid.
2. Check if the email server settings are correct.
3. Check if the intended recipient has an active account.
4. Ensure that the email was sent to the correct address.
5. Check if any rules and filters were applied to your account that prevents emails from being sent/received to specific address.
6. If all of the above are ok but you is still unable to send or receive emails, contact your email service provider for further support.