- Robert via dovecot:
We use a simple system for routing emails to different email users by postfixing the addresses with the actual user: xxxJohn@domain; yyyJohn@domain etc all will be delivered to user John. (This way John can invent a new email address on-the-fly and that will be delivered to his email box.)
This seems like a strange way achieve flexible email addresses. Are you aware of sub-addressing? It has been around for ages, and is supported by Dovecot (and Gmail, incidentally).
Imagine an existing email account alice@example.com. If alice wants to use a subadress, she signs up with alice+foo@example.com, and Dovecot can automatically place incoming mail for that address into INBOX/foo (or just INBOX if INBOX/foo does not exist). Alice can use as many sub-adresses as she needs without anybody making config changes.
Frankly, the Sieve-based approach you describe seems pretty complicated in comparison.
-Ralph