code: @Override
    public void registerMailboxProviderKeeper(final MailboxProviderKeeper keeper) {
      if (this.mailboxProviderKeeper != null) {
        this.mailboxProviderKeeper.close();
      }
      this.mailboxProviderKeeper = keeper;
    }
docs: Registers the {@code MailboxProviderKeeper} plugin.
  @param keeper the {@code MailboxProviderKeeper} to register