Cannot Send email to customer using either mail function or from the back office.

[Memory Nkemjong]

When clients place orders on a virtual product, payment is accepted, a notification mail sent to me on the order but neither the confirmation mail nor the mail with download link is sent to the client. Infact, no email is sent to the client. The status is marked "payment accepted" and when I try to update the status and resend the link under "orders" it shows the mail has been delivered to client but the client never receives anything. 

Am using the php mail function and when I send a test mail there, it goes perfectly well. So in summary, I can receive mails but clients cannot receive mails nor messages from back office.