bp_send_email( string $email_type, string|array|int|WP_User $to, array $args = array() )

function bp_send_email( $email_type, $to, $args = array() ) {
	static $is_default_wpmail = null;
	static $wp_html_emails    = null;

	// Has wp_mail() been filtered to send HTML emails?
	if ( is_null( $wp_html_emails ) ) {
		/** This filter is documented in wp-includes/pluggable.php */
		$wp_html_emails = apply_filters( 'wp_mail_content_type', 'text/plain' ) === 'text/html';
	}

	// Since wp_mail() is a pluggable function, has it been re-defined by another plugin?
	if ( is_null( $is_default_wpmail ) ) {
		try {
			$mirror            = new ReflectionFunction( 'wp_mail' );
			$is_default_wpmail = substr( $mirror->getFileName(), -strlen( 'pluggable.php' ) ) === 'pluggable.php';
		} catch ( Exception $e ) {
			$is_default_wpmail = true;
		}
	}

	$args = bp_parse_args( $args, array(
		'tokens' => array(),
	), 'send_email' );


	/*
	 * Build the email.
	 */

	$email = bp_get_email( $email_type );
	if ( is_wp_error( $email ) ) {
		return $email;
	}

	// From, subject, content are set automatically.
	$email->set_to( $to );
	$email->set_tokens( $args['tokens'] );

	/**
	 * Gives access to an email before it is sent.
	 *
	 * @since BuddyPress 2.8.0
	 *
	 * @param BP_Email                 $email      The email (object) about to be sent.
	 * @param string                   $email_type Type of email being sent.
	 * @param string|array|int|WP_User $to         Either an email address, user ID, WP_User object,
	 *                                             or an array containg the address and name.
     * @param array                    $args {
	 *     Optional. Array of extra parameters.
	 *
	 *     @type array $tokens Optional. Assocative arrays of string replacements for the email.
	 * }
	 */
	do_action_ref_array( 'bp_send_email', array( &$email, $email_type, $to, $args ) );

	$status = $email->validate();
	if ( is_wp_error( $status ) ) {
		return $status;
	}

	/**
	 * Filter this to skip BP's email handling and instead send everything to wp_mail().
	 *
	 * This is done if wp_mail_content_type() has been configured for HTML,
	 * or if wp_mail() has been redeclared (it's a pluggable function).
	 *
	 * @since BuddyPress 2.5.0
	 *
	 * @param bool $use_wp_mail Whether to fallback to the regular wp_mail() function or not.
	 */
	$must_use_wpmail = apply_filters( 'bp_email_use_wp_mail', $wp_html_emails || ! $is_default_wpmail );

	if ( $must_use_wpmail ) {
		$to = $email->get( 'to' );

		return wp_mail(
			array_shift( $to )->get_address(),
			$email->get( 'subject', 'replace-tokens' ),
			$email->get( 'content_plaintext', 'replace-tokens' )
		);
	}


	/*
	 * Send the email.
	 */

	/**
	 * Filter the email delivery class.
	 *
	 * Defaults to BP_PHPMailer, which as you can guess, implements PHPMailer.
	 *
	 * @since BuddyPress 2.5.0
	 *
	 * @param string       $deliver_class The email delivery class name.
	 * @param string       $email_type    Type of email being sent.
	 * @param array|string $to            Array or comma-separated list of email addresses to the email to.
	 * @param array        $args {
	 *     Optional. Array of extra parameters.
	 *
	 *     @type array $tokens Optional. Assocative arrays of string replacements for the email.
	 * }
	 */
	$delivery_class = apply_filters( 'bp_send_email_delivery_class', 'BP_PHPMailer', $email_type, $to, $args );
	if ( ! class_exists( $delivery_class ) ) {
		return new WP_Error( 'missing_class', __CLASS__, $this );
	}

	$delivery = new $delivery_class();
	$status   = $delivery->bp_email( $email );

	if ( is_wp_error( $status ) ) {

		/**
		 * Fires after BuddyPress has tried - and failed - to send an email.
		 *
		 * @since BuddyPress 2.5.0
		 *
		 * @param WP_Error $status A WP_Error object describing why the email failed to send. The contents
		 *                         will vary based on the email delivery class you are using.
		 * @param BP_Email $email  The email we tried to send.
		 */
		do_action( 'bp_send_email_failure', $status, $email );

	} else {

		/**
		 * Fires after BuddyPress has succesfully sent an email.
		 *
		 * @since BuddyPress 2.5.0
		 *
		 * @param bool     $status True if the email was sent successfully.
		 * @param BP_Email $email  The email sent.
		 */
		do_action( 'bp_send_email_success', $status, $email );
	}

	return $status;
}