Mail () problem with some letters

Hi! I have a small problem with my forms. I send it through the mail function $text = $firstname." ".$lastname." ."Firstname ".$firstname."\nLastname: ".$lastname."\...... and so on; mail("mail@mailadress.com","Contactform by $firstname $lastname",$text); When the user has sent the form everything works well and I receive all my informations. But only letters like 'ö' or 'ü' or 'ä' and some more were not displayed correctly. Ie. Instead of 'ö' my mailprogram shows me ö What do I have to do to send the form correctly.