Removes the customer billing email from the Reply-To header on admin order notification emails and replaces it with the store’s configured sender email so replies stay within the store team.

woocommerce-remove-customer-reply-to-admin-emails.php

/**
 * Remove the customer's billing email from the Reply-To header on admin
 * order notification emails (New Order, Cancelled Order, Failed Order)
 * so that replies stay within the store team.
 *
 * Replaces the Reply-To with the store's configured "Email from" address
 * from WooCommerce > Settings > Emails > Email sender options.
 *
 * Requires WooCommerce 3.7+.
 *
 * @since 1.0.0
 *
 * @param string        $header   Email headers.
 * @param string        $email_id Email ID (e.g. 'new_order').
 * @param WC_Order|null $order    The order object, or null if unavailable.
 * @param WC_Email      $email    The email class instance.
 * @return string Modified email headers.
 */
function custom_remove_customer_reply_to( $header, $email_id, $order, $email ) {
	$admin_email_ids = array( 'new_order', 'cancelled_order', 'failed_order' );

	if ( ! in_array( $email_id, $admin_email_ids, true ) ) {
		return $header;
	}

	$header = preg_replace( '/^Reply-to:.*(\r\n|\r|\n|$)/im', '', $header );

	$from_name  = $email->get_from_name();
	$from_email = $email->get_from_address();

	if ( $from_name && $from_email ) {
		$header .= 'Reply-to: ' . $from_name . ' <' . $from_email . ">\r\n";
	}

	return $header;
}
add_filter( 'woocommerce_email_headers', 'custom_remove_customer_reply_to', 10, 4 );