Zimbra Account ID is a
UUID. The mailbox id is an Integer used by the mail server.
The account id is in the accounts.xml file (zimbraId="xxxx"). You can also get it by doing "zmprov getAccount
user@domain.com | grep zimbraId"
You can get the mailbox id by running "zmprov getMailboxInfo
user@domain.com".
When you delete a mailbox, it is gone immediately. You can restore it from backups, and that retention is based on how long you keep your backups laying around.