Sending mail problem.

I have got a answer about sending mail problem like below

For the contact form, ask your hosting provider if they allow sending mail through the php mail function and if there are any email requirements like the sending email must be on the same domain.

I asked my hosting provider and checked our domain allows sending mail throuth the php mail.
but it does not work yet. The hosting provider said mail log is not exist in server even I tested mail.
which file do i have to check and solve this problem?
please help……