Courier can also be configured to pause for a short period of time before attempting to deliver a message. If the mail filter detects a slew of duplicate messages coming in, the mail filter can block all future copies, and manually bounce the handful of copies from the queue. The system administrator can also optionally enable recipient-specified mail filters. With recipient-specified mail filtering enabled, any local mail recipient can install an arbitrary mail filter to selectively accept or reject mail based on any criteria. Currently the mail filtering API is not very well documented, but it's there.
Courier is a closed mail relay by default.
Courier cannot be accidentally configured as a completely open relay. A deliberate feat of stupidity is required for that to happen. Courier also refuses to accept any mail with a return address in a domain with broken MX records. Courier can automatically blacklist domains whose mail servers reject delivery status notifications. Header rewriting Courier will rewrite headers and MIME-ify messages whenever appropriate.