Contact form email is not working

I’m creating contact forms in customer’s webpage. There is a problem with e-mails. For some reason sender’s e-mail addresses do not show in the actual receiver’s e-mail. In other words when somebody fills the form in the webpage and gives his/her e-mail address it does not come through. It is impossible to know where to reply after having received the inquiry from the webpage. What could be the problem?

Looking forward your soonest reply. (Customer is waiting and needs the webpage to work properly).

Regards,

Ilari Savela