Finally got around to upgrading from 10.5.6 to 10.5.7, good news, nothing broke Postfix this time!
Took the opportunity to also upgrade Postfix, now at 2.6.2 here. Nothing tricky about the upgrade, just follow the standard build instructions. Also upgraded Dovecot from 1.13 to 1.2, just released. Again, nothing terribly tricky, just read the docs.