Sending Mail Successfully using PHP mail() but not receiving it

Question

I am sending mail with multiple file attachments. The mail is sent successfully but not receiving in my inbox.

I have checked in my Google Spam Folder and also in All Messages area but i received nothing

<?php
$to = "dummyID@dummy.com";
$subject = "This is subject";
$message = "This is test message.";
$tmp_name = $_FILES['upload_file_array']['tmp_name'];
# Open a file
$file = fopen( $tmp_name, "rb" );
if( $file == false )
{
echo "Error in opening file";
exit();
}
# Read the file into a variable
$size = filesize($tmp_name);
$content = fread( $file, $size);

# encode the data for safe transit
# and insert \r\n after every 76 chars.
$encoded_content = chunk_split( base64_encode($content));

# Get a random 32 bit number using time() as seed.
$num = md5( time() );

# Define the main headers.
$header = "From:xyz@domain.com\r\n";
$header .= "MIME-Version: 1.0\r\n";
$header .= "Content-Type: multipart/mixed; ";
$header .= "boundary=$num\r\n";
$header .= "--$num\r\n";

# Define the message section
$header .= "Content-Type: text/plain\r\n";
$header .= "Content-Transfer-Encoding:8bit\r\n\n";
$header .= "$message\r\n";
$header .= "--$num\r\n";

# Define the attachment section
$header .= "Content-Type:  multipart/mixed; ";
$header .= "name=\"test.txt\"\r\n";
$header .= "Content-Transfer-Encoding:base64\r\n";
$header .= "Content-Disposition:attachment; ";
$header .= "filename=\"test.txt\"\r\n\n";
$header .= "$encoded_content\r\n";
$header .= "--$num--";

# Send email now
$retval = mail ( $to, $subject, "", $header );
if( $retval == true )
{
echo "Message sent successfully...";
}
else
{
echo "Message could not be sent...";
}
?>

Please help me to solve this error...