Good day. You can achieve this by adding this code under line 19 of
wp-content/plugins/woocommerce/templates/emails/email-order-items.php
$terms = get_the_term_list( $_product->id, 'product_cat' );
then under this line of code
<td style="text-align:left; vertical-align:middle; border: 1px solid #eee; word-wrap:break-word;">
add this line
<?php echo ''.$terms.': '; ?>
voila. you got the code.
