code: String mailboxNameFrom(final String candidateMailboxName) {
      if (candidateMailboxName == null) {
        return findDefaultMailboxName();
      } else if (mailboxProviderKeeper.isValidMailboxName(candidateMailboxName)) {
        return candidateMailboxName;
      } else {
        return findDefaultMailboxName();
      }
    }
docs: Answers a {@code name} for a {@code Mailbox} given a {@code candidateMailboxName}, which if non-existing
  the {@code name} of the default {@code Mailbox} is answered. (INTERNAL ONLY)
  @param candidateMailboxName the {@code String} name of the desired {@code Mailbox}
  @return String