Contact form PHP warning

Question

i get the following error when someone tries to send e-mail from the contact form on my website: [06-Jun-2017 10:09:04 UTC] PHP Warning: Cannot modify header information - headers already sent by (output started at /home/../mail/contact.php:31) in /home/../mail/contact.php on line 34

This happens only on the following e-mail: post@domain.com (Using exchange). the contact form works fine with gmail... ?

Source code:

<?php
$name    = $_POST['name'];
$email   = $_POST['email'];
$phone   = $_POST['phone'];
$message = $_POST['message'];

$from    = 'From website';
$headers = "From: post@domain.com\r\n";
$headers .= "Reply-To: post@domain.com\r\n";

if ($_POST['avdeling'] == "avdeling1") {
    $subject = 'To person 1';
    $to      = "Person1@domain.com";
} else if ($_POST['avdeling'] == "avdeling2") {
    $subject = 'To person 2';
    $to      = "Person2@domain.com";
} else if ($_POST['avdeling'] == "avdeling3") {
    $subject = 'To person 3';
    $to      = "Person3@domain.com";
} else if ($_POST['avdeling'] == "avdeling4") {
    $subject = 'To person 4';
    $to      = "Person4@domain.com";
} else{
$subject = 'To post';
$to      = "post@domain.com";
}

$body = "name: $name\n Email: $email\n message:\n $message \n Phone: $phone";
?>

<?php
if (isset($_POST['submit'])) {
    if (mail($to, $subject, $body, $from)) {
        header("Location:domain.com");
    } else {
        echo '<p>Error. Try again!</p>';
    }
}
?>