[Dovecot] Message UID Question

I have referred to the wiki and the RFC, but I still can't find the exact answer I'm looking for. From what I know, Dovecot uses the IMAP standard of 32 bit integers to uniquely identify messages.

After a message is deleted from the mbox, can this UID ever be reissued to a new message or is that UID retired? What about after the 2 billion limit is reached?

Thanks!