email not sending to customer

Hello Everyone,

I am using Opencart Version 1.5.6.3 . Having problem with the contact us form

I am using PHP mail function
Mail Protocol: Mail
Mail Parameters: -Fstoreowner@xyzstore.com

For example my Store URL : http://www.xyzstore.com

First scenario :
Store owner email : storeowner@gmail.com
Customer email : customer@hotmail.com

After submitting the contact us form customer got a success message saying "Your enquiry has been successfully sent to the store owner!". But neither me nor the customer receives the message.

Second scenario :
Store owner email : storeowner@xyzstore.com
Customer email : customer@xyzstore.com

In this case the store owner is receiving message but the customer doesn't.

How can an email be send both to store owner and customer?? I have searched a lot on different forums but none of the solution worked for me. Please help me out ASAP.