GDBM is the recommended library. The file system must support FIFOs. At least the file system that stores the mail queue must be able to support FIFOs. 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.