Filesystem domain sockets must be available. Courier does not implement several legacy features of either MTA, and there are no plans to implement them in the future. Courier expects system mailboxes to be in the users' home directories. Courier's aliases file is also used to implement Qmail-style virtual domains. Courier supports Maildirs natively.
Courier can use the maildrop mail filter as a local mail delivery agent.
Mail filters Courier has hooks for optional, site-defined, mail filters. You'll have to write them yourself, though. 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.