* referring to renamed mailboxes via id.
* @param mailbox The mailbox to cache
* @return The id of the cached mailbox
*/
private Mailbox<Integer> cacheMailbox(Mailbox<Integer> mailbox) {
mailboxCache.add(new MaildirMailbox(mailbox));
int id = mailboxCache.size() - 1;
((MaildirMailbox) mailbox).setMailboxId(id);
return mailbox;
}