function bbp_notify_forum_subscribers( $topic_id = 0, $forum_id = 0, $anonymous_data = array(), $topic_author = 0 ) {

	// Bail if subscriptions are turned off
	if ( ! bbp_is_subscriptions_active() ) {
		return false;
	}

	// Bail if importing
	if ( defined( 'WP_IMPORTING' ) && WP_IMPORTING ) {
		return false;
	}

	/** Validation ************************************************************/

	$topic_id = bbp_get_topic_id( $topic_id );
	$forum_id = bbp_get_forum_id( $forum_id );

	/**
	 * Necessary for backwards compatibility
	 *
	 * @see https://bbpress.trac.wordpress.org/ticket/2620
	 */
	$user_id  = 0;

	/** Topic *****************************************************************/

	// Bail if topic is not public (includes closed)
	if ( ! bbp_is_topic_public( $topic_id ) ) {
		return false;
	}

	// Poster name
	$topic_author_name = bbp_get_topic_author_display_name( $topic_id );

	/** Users *****************************************************************/

	// Get topic subscribers and bail if empty
	$user_ids = bbp_get_subscribers( $forum_id );

	// Remove the topic author from the list.
	$topic_author_key = array_search( (int) $topic_author, $user_ids, true );
	if ( false !== $topic_author_key ) {
		unset( $user_ids[ $topic_author_key ] );
	}

	// Dedicated filter to manipulate user ID's to send emails to
	$user_ids = (array) apply_filters( 'bbp_forum_subscription_user_ids', $user_ids, $topic_id, $forum_id );

	// Bail of the reply author was the only one subscribed.
	if ( empty( $user_ids ) ) {
		return false;
	}

	// Get email addresses, bail if empty
	$email_addresses = bbp_get_email_addresses_from_user_ids( $user_ids );
	if ( empty( $email_addresses ) ) {
		return false;
	}

	/** Mail ******************************************************************/

	// Remove filters from reply content and topic title to prevent content
	// from being encoded with HTML entities, wrapped in paragraph tags, etc...
	bbp_remove_all_filters( 'bbp_get_topic_content' );
	bbp_remove_all_filters( 'bbp_get_topic_title'   );
	bbp_remove_all_filters( 'the_title'             );

	// Strip tags from text and setup mail data
	$blog_name         = wp_specialchars_decode( get_option( 'blogname' ), ENT_QUOTES );
	$topic_title       = wp_specialchars_decode( strip_tags( bbp_get_topic_title( $topic_id ) ), ENT_QUOTES );
	$topic_author_name = wp_specialchars_decode( strip_tags( $topic_author_name ), ENT_QUOTES );
	$topic_content     = wp_specialchars_decode( strip_tags( bbp_get_topic_content( $topic_id ) ), ENT_QUOTES );
	$topic_url         = get_permalink( $topic_id );

	// For plugins to filter messages per reply/topic/user
	$message = sprintf( esc_html__( '%1$s wrote:

%2$s

Topic Link: %3$s

-----------

You are receiving this email because you subscribed to a forum.

Login and visit the topic to unsubscribe from these emails.', 'bbpress' ),

		$topic_author_name,
		$topic_content,
		$topic_url
	);

	$message = apply_filters( 'bbp_forum_subscription_mail_message', $message, $topic_id, $forum_id, $user_id );
	if ( empty( $message ) ) {
		return;
	}

	// For plugins to filter titles per reply/topic/user
	$subject = apply_filters( 'bbp_forum_subscription_mail_title', '[' . $blog_name . '] ' . $topic_title, $topic_id, $forum_id, $user_id );
	if ( empty( $subject ) ) {
		return;
	}

	/** Headers ***************************************************************/

	// Default bbPress X-header
	$headers    = array( bbp_get_email_header() );

	// Get the noreply@ address
	$no_reply   = bbp_get_do_not_reply_address();

	// Setup "From" email address
	$from_email = apply_filters( 'bbp_subscription_from_email', $no_reply );

	// Setup the From header
	$headers[] = 'From: ' . get_bloginfo( 'name' ) . ' <' . $from_email . '>';

	// Loop through addresses
	foreach ( (array) $email_addresses as $address ) {
		$headers[] = 'Bcc: ' . $address;
	}

	/** Send it ***************************************************************/

	// Custom headers
	$headers  = apply_filters( 'bbp_subscription_mail_headers', $headers  );
	$to_email = apply_filters( 'bbp_subscription_to_email',     $no_reply );

	// Before
	do_action( 'bbp_pre_notify_forum_subscribers', $topic_id, $forum_id, $user_ids );

	// Send notification email
	wp_mail( $to_email, $subject, $message, $headers );

	// After
	do_action( 'bbp_post_notify_forum_subscribers', $topic_id, $forum_id, $user_ids );

	// Restore previously removed filters
	bbp_restore_all_filters( 'bbp_get_topic_content' );
	bbp_restore_all_filters( 'bbp_get_topic_title'   );
	bbp_restore_all_filters( 'the_title'             );

	return true;
}