Solution:
- Check email settings: In cPanel, go to Email Accounts and verify your email account settings. Ensure SMTP and POP/IMAP settings are correct.
- Check DNS settings: Ensure that the MX, SPF, and DKIM records are configured properly in Zone Editor.
- Check email quotas: Go to Email Accounts to see if any of your accounts have exceeded storage limits and adjust as needed.
- Check for blacklisting: Use online tools to check if your IP/domain is blacklisted. If it is, contact your provider to resolve the issue.