code: @Nonnull
    public static EChange setMailQueueSize (@Nonnegative final int nMaxMailQueueLen,
                                            @Nonnegative final int nMaxMailSendCount)
    {
      ValueEnforcer.isGT0 (nMaxMailQueueLen, "MaxMailQueueLen");
      ValueEnforcer.isGT0 (nMaxMailSendCount, "MaxMailSendCount");
      ValueEnforcer.isTrue (nMaxMailQueueLen >= nMaxMailSendCount,
                            () -> "MaxMailQueueLen (" +
                                  nMaxMailQueueLen +
                                  ") must be >= than MaxMailSendCount (" +
                                  nMaxMailSendCount +
                                  ")");
  
      return s_aRWLock.writeLocked ( () -> {
        if (nMaxMailQueueLen == s_nMaxMailQueueLen && nMaxMailSendCount == s_nMaxMailSendCount)
          return EChange.UNCHANGED;
        s_nMaxMailQueueLen = nMaxMailQueueLen;
        s_nMaxMailSendCount = nMaxMailSendCount;
        return EChange.CHANGED;
      });
    }
docs: Set mail queue settings. Changing these settings has no effect on existing
  mail queues!
  
  @param nMaxMailQueueLen
  The maximum number of mails that can be queued. Must be &gt; 0.
  @param nMaxMailSendCount
  The maximum number of mails that are send out in one mail session.
  Must be &gt; 0 but &le; than {@link #getMaxMailQueueLength()}.
  @return {@link EChange}.