If you're customers emails are being sent to your spam folder, then you can whitelist that email address OR domain by following these steps:

1. Login to your cPanel hosting account via here.
   
   
2. Open MAILSCANNER
   
3. Open WHITELIST/BLACKLIST Settings
   
4. Enter the email or domain here you wish to whitelist:
   

   5. Don't forget to save by clicking the CHANGE button.