multiple checkboxes in an email enquiry form

Hey there, I have a question - I'm trying to send a contact form and if the user has items in their account they can click multiple checkboxes and it will send the information in the email along with the item names and links

My code:

<?php if ( is_user_logged_in() && have_posts() ) : ?>
<label><?php printf( __( "%s's Item's", APP_TD ), $display_user_name ); ?></label><br>
<?php 
while( have_posts() ) : the_post(); $i++; ?>
<input type="checkbox" name="item_offer" id="item_offer" value="<?php the_title(); ?>"><a href="<?php the_permalink(); ?>"><?php the_title(); ?></a>
<?php endwhile; ?>
<?php else : ?>
<p class="text-center"><?php _e( 'Please login to select your items.', APP_TD ); ?></p>
<?php endif;?>

Now the items show up in the contact form and the contact form sends, but it only shows 1 item checked and not 2 if 2 are select or 3.. it only shows the 1

email.php

$item_offer = appthemes_filter( appthemes_clean( $_POST['item_offer'] ) );

$message .= $item_offer . "\r\n";

How do i display in the email all the checkboxes the user may tick?

I hope someone can help me

Thank You