Tienes q añadir el charset.
Prueba con esto:
Código PHP:
$headers = "From: $email";
$headers .= "MIME-Version: 1.0\r\n";
$headers .= "Content-type: text/plain; charset=iso-8859-1\r\n";
mail($emailnewmember,
"$sitetitle Member Account",
"Hello,\n\nYou have registered to $sitetitle!\nNow you must activate yor account by visiting the following link: http://web/activate.php?user=$emailusername&email=$emailnewmember&status=1\n(if the link doesn`t work please copy and paste it in your browser).\n\nYour account information:\nUsername: $emailusername\nPassword: $emailpassword\n\nThank you for your interest!\n\nBest Regards!\n\n\nThis message isn`t a SPAM, you are receiving this message because you have signup with $sitetitle\nIf you didn`t created an account and you may think that somebody else has done that for you, report this problem by contacting us: $email",
$headers);