When updating cyrus imapd server from v2.1+ to v2.3.3 started looking back for these local small patches:

• luser_relay patch - accept mail adressed to non-existing users and deliver it to another existing account (like postmaster) - useful to stop fantom bouncings (attached at the end on this page, for FreeBSD first drop it into the files dir (e.g. /usr/ports/mail/cyrus-imapd23/files/) then do the usual make && make install thing)