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<?php

// Product Enquiry Form: send email to post author instead of Site Admin
add_filter( 'product_enquiry_send_to', 'wc_product_enquiry_to_author', 10, 2 );

function wc_product_enquiry_to_author( $email, $product_id ) {

    $post_author_id = get_post_field( 'post_author', $product_id );
    $post_author_email = get_the_author_meta( 'user_email', $post_author_id );

    return $post_author_email;
}