El código del formulario:
<form action="sendmail_auto.php" method="POST" class="details_form" id="offer_send_friend">
<div class="form_col_1">
<div class="row">
<label class="label_title">Tu email:</label>
<input type="text" name="email" id="email" class="inputField required" value="">
</div>
<div class="row">
<label class="label_title">Destinatario:</label>
<input type="text" name="email_f" id="email_f" class="inputField required" value="">
</div>
</div>
<div class="form_col_2 col_thin">
<div class="row">
<label class="label_title">Mensaje:</label>
<textarea rows="4" cols="5" name="message" id="message" class="textareaField required">Este vehículo te puede interesar:
http://autopartesacr.com.mx/detallesCar.php?idproducto=<?php echo"$idauto" ?></textarea>
</div>
<div class="row rowSubmit">
<a href="#" class="link_reset" onclick="document.getElementById('offer_send_frien d').reset();return false">Resetear Info.</a>
<input type="submit" value="ENVIAR MENSAJE" id="send_f" class="btn_send">
</div>
</div>
<div class="form_col_3">
<div class="row">
<label class="label_title">Share on:</label>
<span class='st_sharethis_vcount' displayText='ShareThis'></span>
<span class='st_facebook_vcount' displayText='Facebook'></span>
<span class='st_twitter_vcount' displayText='Tweet'></span>
</div>
</div>
</form>