PHP @Mail总是设置为false

Question

Hi my php mail functions always results false i have even made the call function mail() on the code but it remains then altered the code by declaring a variable $f and still no good

    <?php

$message="";
$f="";
$mail_sent="";
if(isset($_POST['submit']))
{
    $headers = "MIME-Version: 1.0" . "
";
    $headers .= "Content-type:text/html;charset=iso-8859-1" . "
";
    $headers .= "From:".$_POST['email']."
";


    $recipient="[e-mail]";


    $subject = "CONTACT MAIL LETTER
";
    $message .= "<span style='font-family: verdana; font-size: 12px;'>Dear Sir, <br>&nbsp;&nbsp;&nbsp;&nbsp;"."<br> Contact Us details are displayed below <br><br>";   

    $message.='<tr><td><b><u>CONTACT US DETAILS</u></b></td><td></td><td></td></tr>';

    $message.="<BR><BR><b>FIRST NAME:</b> ".$_POST['fname'].  "<BR><BR>";

    $message.="<BR><BR><b>LAST NAME:</b> ".$_POST['lname'].  "<BR><BR>"; 

     $message.="<b>E-MAIL ADDRESS:</b> ".$_POST['email'].  "<BR><BR>";

     $message.="<b>SUBJECT:</b> ".$_POST['subject'].  "<BR><BR>";   

     $message.="<b>MESSAGE:</b> ".$_POST['message'].  "<BR><BR><BR><BR><BR><BR><BR>";

     $message .= "Thanks,<BR>".$_POST['fname']. "</span>";
     //echo $message;


    $mail_sent = @mail($recipient,$subject,$message,$headers);
    if($mail_sent)
    {
    $f=1 ;
    }
    else
    {
    $f=0;
    }
    }

<?php 

if(isset($_POST['submit']))
{

if($f==1)
         {
            echo "Your Mail has been sent Successfully.";
          }
       else
       {
        echo "Your Mail has not been sent Successfully.";
       }
}
?>

The above functions always echo unsuccsfull