Pa nije ni cudo da ti ne radi kad nisi stavio nijdno string polje unutar navodnika. Dakle, treba:
Kôd:
$query = "INSERT INTO Client (fname, lname, email, address, tel, country, zip)
VALUES ('$custom_First_Name', '$custom_Last_Name', '$from', '$custom_Address',
'$custom_Phone_Number', '$custom_Country', '$custom_Post_Code' ";