I'm close to understanding it, but one thing in documentation makes me unsure.
At first, lets take a look at postfix architecture from docs. There are two stages: receiving and delivering. During receiving stage mail is stored in incoming queue and smtp client is notified about successfully enqueueing.
Also documentation says relocated_maps is treating by trivial-rewrite ONLY during delivering stage. And at same time it says:
As of Postfix version 2, mail for a relocated user will be rejected by the SMTP server with the reason "user has moved to firstname.lastname@example.org". Older Postfix versions will receive the mail first, and then return it to the sender as undeliverable, with the same reason.
How is it possible to reject receiving mail during delivering stage?