PHP Mail function not working on Web Services - php

Question

I am using two different servers for dev site and live site. My webservice function to send email confirmation is working on Dev Site but it is not working in Live Site. My
email confirmation function is:
function getEmailConfirmation($UserName, $ChecklistID,$UserID,$AnswerBulkID, $TotalSubmitted){
if ( ($UserID == '') || (!isset($UserID)) ) {
return error("partial");
}
$entry = new Entry();
$entry_date_check = date('Y-m-d'); //for check if already
$count_tbl_answers = $entry->listEntry("SELECT count(*) as total_from_answers FROM tbl_answers where user_id='".$UserID."' and answer_bulk_id ='".$AnswerBulkID."' and date(ent_date) = '".$entry_date_check."' ");
$to = "****#gmail.com";
$subject = "TDL Checklist email";
$message = "Hello there! \n User ID: ".$UserID." \n User Name: ".$UserName."\n Check List ID: ".$ChecklistID."\n Answer Bulk ID: ".$AnswerBulkID;
$from = "TLD-Email-Confirmation";
$headers = "From:" . $from;
if(($count_tbl_answers[0]['total_from_answers']) == $TotalSubmitted){
mail($to,$subject,$message,$headers);
$result = array(
"user_id" => $UserID,
"user_name" => $UserName,
"check_list_id" => $ChecklistID,
"answer_bulk_id" => $AnswerBulkID,
);
} else{
$delete_bulk = $entry->query("delete FROM tbl_answerbulk WHERE user_id ='".$UserID."' and check_list_id ='".$ChecklistID."' and id ='".$AnswerBulkID."' ");
$delete_answer = $entry->query("delete FROM tbl_answers WHERE user_id='".$UserID."' and answer_bulk_id ='".$AnswerBulkID."' ");
return error("custom","","Could not submit your answer this time. Please try again later.");
}
$message = json_encode(
$result
);
$fin_data['response'] = json_encode(array('Message'=>'ok','Data'=>$message));
return $fin_data;
}
This function works perfectly on dev server, but it does not work on live server. What might be the reason?
Note:
Although mail function not working on Live site, it returns the result array which is just after the mail function.
That is:
#mail($to,$subject,$message,$headers);
$result = array(
"user_id" => $UserID,
"user_name" => $UserName,
"check_list_id" => $ChecklistID,
"answer_bulk_id" => $AnswerBulkID,
);
I also tried ini_set('smtp_port', 587); and #mail($to,$subject,$message,$headers); but does not worked.