Add ability to throttle mail sending

    /**
     * @return the number of mails to send before waiting {@link #getPacketDelay()} milliseconds before starting the
     *         next packet. Used to implement email Throttling.
     */
    int getPacketSize();

    /**
     * @return the delay in milliseconds to wait between each packet of {@link #getPacketSize()} mails sent before
     *         starting the next packet. Used to implement email Throttling.
     */
    long getPacketDelay();