docs: public void removeMailbox(long hsId) {
          synchronized (m_mapLock) {
              ImmutableMap.Builder<Long, Mailbox> b = ImmutableMap.builder();
              for (Map.Entry<Long, Mailbox> e : m_siteMailboxes.entrySet()) {
                  if (e.getKey().equals(hsId)) {
                      continue;
                  }
                  b.put(e.getKey(), e.getValue());
              }
              m_siteMailboxes = b.build();
          }
      }