Order confirmation email not sending to customer

[Amatory]

Hi,

 

I seem to have an issue that i can't find a clear fix on.

 

So when customers order it should send them and myself an email, however we are getting the email but the customer IS NOT getting the order confirmation email.

 

I am using PS 1.6.1.4

Mal alerts are on.

I am using -  PHP's mail() function

 

Any help would be great,

Thank you in advance.