Email with PHP mail function with Ajax is bouncing when message exceed limit 1599 characters

Question

I am having issue of bounce email in PHP.

Email with PHP mail function with AJAX is bouncing when message exceeds 1599 characters limit.

$.ajax({
     url: 'send_renewal_email.php',
     type: 'post',
     data: { ids: checkValues },
     success:function(data){
                $("#LoadingImage").hide();
                $('input:checkbox').attr('checked',false);
                $('#email_sent_success').show().delay(5000).fadeOut();  
                $('#email_sent_success').html(data);
                window.location.reload();
     }
});

If $message has more then 1599 characters then mail bounces. Please help me in this matter. I tried to found solution but not succeed.

mail($to,$subject,$message,$headers)